Position a rack in the center of the oven and preheat oven to 350° F. Butter and flour two 9-inch round baking pans and set them aside.

In a medium bowl, whisk together the flour, baking powder and salt. In a separate medium bowl, whisk together the buttermilk and egg whites.

In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar until they are light and creamy, about 3 minutes.

Add the vanilla and scrape down the sides of the bowl; beat again.

Add one-third of the flour mixture, still beating on medium speed.

Beat in half of the milk-egg mixture, then beat in half of the remaining dry ingredients until incorporated.

Add the rest of the milk-egg mixture, beating until the batter is smooth and then add the last of the dry ingredients. Finally, give the batter a good 2-minute beating to ensure that it is thoroughly mixed and well aerated. Divide the batter evenly into the prepared baking pans and smooth the top with a rubber spatula. Bake for 35-40 minutes or until a toothpick inserted in the center comes out clean.Let cakes cool completely in the pans on a wire rack, at least one hour.

Remove cakes from pans and place on a piece of plastic wrap.

In a medium saucepan over medium-low heat, cook the blueberries, 3/4 cup water, and 2 Tbsp sugar, partially covered, until blueberries are softened, about 8 minutes.

Strain through a fine-mesh sieve set over a medium bowl and discard solids.

Whisk berry-flavored gelatin and food coloring into the juices and cool slightly, about 15 minutes.

In another medium saucepan over medium-low heat, cook the strawberries, 1/2 cup water and 2 Tbsp sugar, partially covered, until berries are softened, about 15 minutes.

Strain through a fine-mesh sieve set over a medium bowl and discard solids.

Whisk strawberry gelatin into the juices and cool slightly, about 15 minutes.

Using a skewer, poke 25 holes in the top of each cake, twisting gently to form larger holes. Pour cooled blueberry syrup over one cake layer.

Repeat with cooled strawberry syrup and second cake layer.

Cover cakes with plastic wrap and refrigerate until gelatin is set, at least 3 hours or up to 24 hours. I put mine in the freezer overnight; I think frozen cake layers are much easier to frost.

Make the whipped cream stabilizer by adding the gelatin to the cold water and letting it sit for a minute. Microwave on high for just long enough to become liquid; mine took about 10 seconds then let cool for one minute.

In a large chilled bowl, beat cream and sugar until soft peaks form; while mixer is running add stabilizer and mix thoroughly to incorporate. Refrigerate until ready to use.

Place blueberry cake, syrup side up, on a platter. Spread 1 cup of whipped cream over top.

Place strawberry cake layer, syrup side up, on top of whipped cream.

Spread remaining whipped cream over top and sides of cake.

Garnish with additional blueberries and strawberries.