For the crust

1. Lightly grease a 9-inch round fluted tart pan with a removable bottom.
2. In a small saucepan over medium heat, melt the butter and continue to cook until it caramelizes and emits a nutty, butterscotch aroma. You will notice brown flecks in the bottom of the pan as the butter deepens in color. This should take about 3-6 minutes. Be careful not to burn the butter. Remove from the heat.
3. In a medium bowl, combine the melted butter with the brown sugar, vanilla and salt. Add the flour and mix until incorporated. Spread the dough evenly and up the sides of the prepared pan. Let stand for at least 1 hours or up to overnight (do not refrigerate).
4. Preheat oven to 300°F.
5. Bake the shortbread for 45 minutes. Remove the pan from the oven. Let cool completely.

For the filling

1. Place the lemon curd in a medium bowl. Set aside.
2. Place your metal bowl and whisk attachment or beaters in the freezer for about ten minutes to chill. Remove from the freezer. When chilled, add the heavy cream. In the stand mixer with a whisk attachment, whip the cream on low speed for about 30 seconds.
3. Increase the speed to high and whip until soft peaks form, 2-3 minutes. Add half of the whipped cream to the bowl with the lemon curd and fold in gently. Once combined, add in the rest of the whipped cream and fold it in. Pour the creamy lemon mixture into the cooled crust and spread evenly.
4. Freeze for at least two hours before serving. Keep stored in the freezer for up to 3 weeks.