For the Cake:

Cup of Yum
1. Preheat the oven to 350F. Prepare a 13" x 9" sheet cake pan by adding parchment paper.
2. Sift flour, baking powder, salt and granulated sugar. Give it a whisk and set aside.
3. Add egg whites, milk, vanilla, almond extract, and sour cream. Whisk together.
4. Attach a paddle attachment to the stand mixer. Add in room temperature butter and beat until creamy. 
5. On medium speed, beat for the dry into the butter until you get a crumby mixture. Add 1/2 of the wet mixture. Mix, then scrape the bowl down and add the remaining. Beat until combined. 
6. Cut the raspberries in half. Squeeze a little bit of lemon juice over the berries. Add some flour and toss until they're coated.  Gently fold the raspberries and blueberries into the batter by hand.
7. Pour the batter into the prepared sheet cake pan.
8. Throw in a few more berries. 
9. Bake for 35- 40 minutes and/or until a toothpick inserted comes out clean.
10. Let the cake cool to room temperature on a cooling rack.

For the Frosting:

1. In a stand mixer fitted with a whisk attachment, add heavy whipping cream and confectioners sugar.
2. Beat on high until whipped.
3. Add vanilla extract. Give it another whip on high.
4. Fold in mascarpone by hand using a spatula. Switch to a whisk and beat.

For the Assembly:

1. I trimmed the sides of the cake. You don't have to do this, but I wanted a nice fresh edge.
2. Use an ice cream scooper to add the frosting.
3. Smooth with an offset spatula.
4. Using a ruler, I sketched out my flag, then transferred the design to the buttercream.
5. If you’re making an American flag, add blueberries to the top left side of the cake. Add raspberries in lines. Repeat until the flag is finished.