Whisk yolks and 1/4 cup sugar until thick and creamy (ribbon stage) then continue whisking over a double boiler until fully warmed through. Add 1 tsp flavoring.
Clean beaters. In separate bowl, whisk egg whites until frothy, then sprinkle in remaining 1/4 cup sugar gradually while whisking, until fully incorporated. Add 1/2 tsp flavoring and continue mixing until you reach stiff peaks.
In another separate bowl, whisk cold heavy cream until thickened. Add remaining 3 Tbsp sugar and 1/2 tsp flavoring, continue mixing until you reach stiff peaks.
Fold meringue (egg white mixture) into base (egg yolk mixture) in two portions. Repeat with heavy cream, until fully incorporated with no white streaks. Pour into bowls, springform pan, or plastic-lined mold, place in freezer.
Repeat steps 1-4 for each new layer of semifreddo you want to add to cake, flavoring and coloring as desired. Pour each new semifreddo layer onto previously frozen layer, to create separate layers/flavors/colors.
For shortbread base, melt butter and add to the rest of the ingredients. Mix until fully incorporated. Shape into a large disk about an inch smaller than your mold. Dock with fork like you would with a pie crust. Baking at 400 until golden brown. Allow to come to room temperature, depan, wrap in plastic and freeze.
Freeze top layer (if doing more than one) semifreddo mixture for about 1 hour, then press the frozen shortbread disk into the surface. Smooth edges with spatula or butter knife, then freeze overnight.