Preheat oven to 350°F. Line an 8x8" pan with aluminum foil or parchment paper and spray with cooking spray; set aside.

In a large microwavable bowl, melt the butter in the microwave. Whisk in the brown sugar. Add the egg and vanilla extract and whisk until smooth.

Add the flour and salt and fold until combined. Add the almonds, cashews, and pecans and fold in. Pour batter into prepared pan, smoothing it lightly with a spatula or offset knife.

Bake for 18-20 minutes, or until the edges are golden brown. Let cool completely before cutting into 16 squares.