To make the cake: Preheat oven to 350 F. Spray two 9-inch round cake pans with nonstick cooking spray with flour.
In a large measuring cup, whisk together the milk, egg whites and vanilla. Combine the flour, sugar, baking powder, and salt in the bowl of a stand mixer fitted with the paddle attachment. Beat on low briefly to combine. With the mixer still on low, add the butter, a few pieces at a time, and continue beating until the pieces of butter are about the size of peas.
Add all but 1/2 cup of the milk mixture to the mixer and beat on medium speed for 1 1/2 minutes. The batter should be smooth. Add the remaining 1/2 cup of milk mixture and beat for an additional 30 seconds. Scrape down the sides of the bowl, then beat for 20 seconds longer on medium.
Divide the batter between the prepared pans. Use an offset spatula to spread into an even layer. Bake for 22-25 minutes, or until a toothpick inserted in the center comes out clean. Transfer the pans to a wire rack and allow the cakes to cool completely in the pans.
Meanwhile, make the blueberry and strawberry syrups. To make the blueberry syrup: In a medium saucepan, combine the blueberries, water, and sugar. Set the pan over medium-low heat, cover, and cook until the blueberries are softened – about 8-10 minutes. Keep a close eye, it can boil over quickly if you turn your back. Pour the mixture through a fine-mesh strainer into a heatproof bowl. Discard the solids. Whisk the gelatin into the liquid until dissolved. Let cool for at least 15 minutes. To make the strawberry syrup: Repeat the exact same process (combine ingredients, cook and strain) then whisk in the gelatin and again, let cool for at least 15 minutes.
When the cakes are cool, use a wood skewer to poke 25 holes in the top of each cake. You want holes all over the cakes but try not to poke all the way down if you can help it. (A few of my "pokes" always get to the bottom. No problem.) Slowly pour the blueberry syrup over the top of one cake, and the strawberry syrup over the top of the other. Cover both cake pans with plastic wrap and refrigerate until the gelatin is set, at least 3 hours, or up to 1 day.
Right before you’re ready to assemble, make the whipped cream: Combine the heavy cream, sugar and vanilla in the bowl of a stand mixer fitted with the whisk attachment. Starting on low and increasing the speed to medium-high, beat until the whipped cream holds stiff peaks.
Turn both cakes out of the pans (you may need to run a thin knife along the edges to loosen it. If it's stubborn, set the pan in warm water up to 1 minute). Place the blueberry cake, right side up, on your serving plate. Spread 1 cup of whipped cream over the cake, then place the strawberry cake on top (again, right side up). Use the remaining whipped cream to cover the top and sides of the cake. Garnish with blueberries and strawberries, if desired. Store the cake in the refrigerator. This can be served the day of assembly or even the following day. Store leftovers covered in refrigerator for up to 3 days.