Sift cake flour, baking powder, and salt into a large bowl. Add sugar and whisk to combine.
In a separate bowl, whisk egg whites, milk, vanilla extract, almond extract, and sour cream until fully incorporated.
Using a stand mixer with paddle attachment, cream room temperature butter until smooth and airy. Gradually beat in dry ingredients until crumbly texture forms.
Add the wet ingredients to the butter and dry mixture, mixing until just combined. Pour batter into a 13×9-inch greased and floured baking pan.
Bake at 350°F (175°C) for 35–40 minutes or until a toothpick comes out clean. Allow to cool completely.
Trim edges for a clean work surface. Dust raspberries and blueberries with a 1:1 cornstarch-sugar mix to prevent sinking.
Pipe mascarpone buttercream (mix 8 oz mascarpone with 2 tbsp powdered sugar and 1 tsp vanilla) in a J-shaped flag pattern. Arrange berries to form stars and stripes as desired. Chill before serving.