Make the cake layers (up to 4 days ahead):
Preheat the oven to 425 degrees. Butter two 9-inch spring form or layer cake pans.
Cream butter and sugar in a mixer until light and fluffy. Add the eggs one at a time, beating thoroughly after each one.
Combine flour, baking powder and salt in a bowl. Combine buttermilk and vanilla separately. With the mixer running on low speed, add some of the dry ingredients and beat until just incorporated. Add some of the buttermilk mixture and again beat until incorporated. Repeat three or four times until both are used up.
Pour the batter into the prepared pans and smooth the top, forming a slight concave dip in the center. (The cake will rise more in the center when baked). Bake 15 minutes, rotate the pans in the oven and bake until a cake tester is clean and the edges just start to pull away from the side of the pan; another 15 minutes. Remove from oven and let cool completely before wrapping to store or assembling the cake.
Make the filling (the night before):
Whip the cream until it has thickened but soft peaks have not yet formed. Add the sugar and whip to soft peaks.
Cut 14 evenly-sized strawberries in half lengthwise. Cut 4-5 strawberries into thin lengthwise slices and set aside with 2 tablespoons of the blueberries to garnish the top of the cake.
Place one cake layer on your serving plate. Place halved strawberries, cut end facing out and flat edge down, around the perimeter of each cake layer, alternating with blueberries. Chop the remaining whole strawberries into 1/4-inch pieces, combine with the blueberries and fold into the whipped cream.
Assemble the cake (2-3 hours before the party):
Spoon about 3/4 of the whipped cream on the bottom layer and spread evenly to the edges. Top with the second layer and spoon the remaining whipped cream into a 6-inch circle in the center of the top layer. Arrange the reserved sliced strawberries and blueberries decoratively on top of the whipped cream.