Preheat the oven to 350 degrees Fahrenheit and grease and line a 7 inch round cake pan. Cut the butter into cubes and beat it with the brown sugar until combined.

In a separate bowl, sift together the all purpose flour, salt, baking soda, baking powder, and cocoa powder.

Add the egg yolks, vegetable oil, lemon juice, and vanilla to the butter and sugar. Beat for 1 to 2 minutes until combined.

Add the red food coloring and beat until the color is even and no lumps remain.

Add the buttermilk and the sifted dry ingredients and stir until completely combined with no dry streaks.

In a clean bowl, beat the egg whites on high speed for 4 to 5 minutes until stiff peaks form.

Gently fold the whipped egg whites into the batter until no white streaks remain.

Pour the batter into the prepared pan. Cover loosely with foil for the first 20 minutes so the top does not brown too fast, then uncover and bake until a wooden skewer comes out with only a few moist crumbs, about 50 to 60 minutes total. For an even bake, divide the batter into two batches, two thirds and one third, and bake them one after the other.

Rest the cake in the pan for 10 minutes, then turn it out and cool completely. Trim the darker top and bottom crusts and slice the cake into 3 even layers. Save the trimmings for crumbs.

Beat the butter and mascarpone on medium speed for 1 to 2 minutes until combined. Add the vanilla bean paste and powdered sugar and beat 1 to 2 minutes more until smooth.

Spread frosting over each layer and stack. Cover the top and sides with the remaining frosting and press the reserved cake crumbs over the top or sides to decorate.

Refrigerate the cake for 2 to 3 hours before slicing so the frosting sets and the layers hold together cleanly.