Preheat oven to 350°F. Grease two 9-inch round cake pans with butter; dust pans with flour and knock out excess. Combine chopped chocolate, cocoa powder, and hot water in medium heatproof bowl; set bowl over saucepan containing 1 inch of simmering water and stir with rubber spatula until chocolate is melted, about 2 minutes. Add 1/2 cup sugar to chocolate and stir until glossy, 1-2 minutes. Remove bowl from heat and set aside to cool.

Whisk flour, baking soda, and salt in medium bowl. Combine buttermilk and vanilla in small bowl. In bowl of stand mixer fitted with whisk attachment, whisk eggs and yolks on medium-low speed until combined, about 10 seconds. Add remaining 1 1/4 C sugar, increase speed to high, and whisk until fluffy and lightened in color, 2-3 minutes. Replace whisk with paddle attachment. Add cooled chocolate mixture to egg/sugar mixture and mix on medium speed until thoroughly incorporated, 30-45 seconds, pausing to scrape down sides of bowl with rubber spatula as needed. Add softened butter 1 tablespoon at a time, mixing about 10 seconds after each addition. Add about 1/3 of flour mixture; followed by half of buttermilk mixture mixing until incorporated after each addition (about 15 seconds.) Repeat using half of remaining flour mixture and all of remaining buttermilk mixture (batter may appear separated.) Scrape down sides of bowl and add remaining flour mixture; mix at medium low speed until batter is thoroughly combined, about 15 seconds. Remove bowl from mixer and fold batter once or twice with rubber spatula to incorporate any remaining flour.

Divide batter evenly between prepared cake pans; smooth batter to edges of pan with spatula. 3. Bake cakes until toothpick inserted into center comes out with a few crumbs attached, 25-30minutes. Cool cakes in pans 15 minutes, then invert onto wire rack. Cool cakes to room temperature before frosting, 45-60 minutes.

Preheat oven to 350*F. Line two 9-inch pans with parchment paper, and grease with butter and flour or Pam.

Cream the butter and the sugars until light and fluffy. Scrape down the side of the bowl. Continue mixing while adding the egg. Add the vanilla. Scrape down the bowl with a spatula. Combine the flour, soda and salt in another bowl. With a whisk, stir to combine. With the machine on low, slowly add the flour. Mix until just combined, taking care not to over mix. With a spatula, fold in the chocolate.

Divide the dough into the two prepared pans and spread the dough evenly. Bake at 350* for 12-15 minutes. They should be lightly golden on the outside and just set in the middle. Let cool in pans for 15 minutes, and then transfer to a wire rack to cool completely.

Line a 9-inch pan with plastic wrap and spray with a non-stick baking spray. Set aside. Cream the butter and the sugars until light and fluffy. Scrape down the side of the bowl. Add in the milk and vanilla and mix to combine. Scrape down the bowl with a spatula.

Combine the flour and salt in another bowl. With a whisk, stir to combine.

With the machine on low, slowly add the flour. Mix until just combined, taking care not to over mix. With a spatula, fold in the chocolate.

Spread the dough into the prepared pan and place in the freezer until ready to use.

Melt chocolate in heatproof bowl set over a saucepan containing 1 inch of barely simmering water, stirring occasionally until smooth. Remove from heat and set aside. Meanwhile, heat butter in small saucepan over medium-low heat until melted. Increase heat to medium; add sugar, corn syrup, vanilla, and salt and stir with heatproof rubber spatula until sugar is dissolved, 3-4 minutes. Add melted chocolate, butter mixture, and cream to clean bowl of stand mixer and stir to thoroughly combine.

Place mixer bowl over ice bath and stir mixture constantly with rubber spatula until frosting is thick and just beginning to harden against sides of bowl, 1-2 minutes (frosting should be 70 degrees).

Place bowl on stand mixer fitted with paddle attachment and beat on medium-high speed until frosting is light and fluffy and has lightened in color, 3-4 minutes. Stir with rubber spatula until completely smooth.

*If you find after beating, your frosting is still too soft for your liking, try chilling in the fridge until it sets up a little more.

With a hand mixer or standing mixer fitted with the paddle attachment, beat butter for 4 minutes on medium speed ("4" on my KitchenAid) until smooth. Add powdered sugar and mix on low until fully incorporated.

Add melted chocolate and beat until combined. Add marshmallow cream and beat until frosting has lightened in color and all ingredients are well combined, about 3 minutes.

Place one of the cooled cookie layers on a cake plate or cake board. Add about 1 cup of the dark chocolate frosting to the top of the cookie and spread evenly with an offset palette knife. Place one layer of chocolate cake on top of the frosted cookie. Add another cup of frosting and spread evenly, creating a flat layer. Remove the cookie dough from the freezer and unwrap, and then place on top of the frosted cake layer. Add another cup of frosting and spread evenly. Next, place the second layer of chocolate cake on top. Spread one more cup of frosting, and top with remaining cookie, placing it top side down to create a flat surface on top.

Place a generous scoop of the milk chocolate marshmallow frosting on top, spreading evenly with a small offset palette knife and working your way down the sides until you have a thin layer of frosting over the entire cake - a crumb coat. Chill until set — about 30 minutes. Frost the cake however you would like, and garnish with chocolate chips, chocolate sprinkles, and marshmallows! Serve at room temperature.