Preheat the oven to 350°F (175°C) and prepare three 9-inch round cake pans with parchment paper and non-stick spray.
In a large mixing bowl, cream together the softened butter and sugar until light and fluffy, about 5 minutes.
Add eggs one at a time, mixing well after each addition. Then, blend in the vanilla extract.
In a separate bowl, whisk together the flour, cocoa powder, and baking powder.
Alternate adding the dry mixture and buttermilk to the creamed mixture, beginning and ending with the dry ingredients. Mix until just combined.
Divide the batter evenly into three bowls. In one bowl, mix in the strawberry puree; in the second, the cocoa powder for the chocolate layer; leave the third as vanilla.
Pour the vanilla batter into one prepared pan, chocolate into another, and strawberry into the last. Bake for 25-30 minutes, until a toothpick inserted comes out clean.
Once baked, turn the cakes onto a wire rack to cool completely before frosting.
Whip the heavy cream until stiff peaks form, then divide it into three portions: leave one plain, mix cocoa powder into one, and strawberry puree into the other.
Place the vanilla layer on a serving plate, spread a layer of vanilla frosting on top, then add the strawberry layer, followed by the strawberry frosting. Repeat with the chocolate layer and chocolate frosting.
Finish by frosting the entire cake with any remaining frosting and chilling it for at least an hour before serving.