Preheat your oven to 350°F (175°C).
Grease and flour three 9-inch round cake pans.
In a large bowl, cream together the softened butter and sugar until light and fluffy.
Add eggs, one at a time, beating well after each addition.
Alternate adding the dry ingredients (flour, cocoa powder, baking powder, and salt) and milk, mixing until just combined.
Divide the batter into three bowls, adding strawberry puree to one, mixing well, and leaving the other two as vanilla and chocolate.
Pour each flavor into its prepared pan and bake for 25-30 minutes, or until a toothpick comes out clean.
Let the cakes cool in the pans for 10 minutes, then transfer to wire racks to cool completely.
Once cooled, level each cake if necessary, ensuring even layers for stacking.
Frost each layer with buttercream, stacking them accordingly.
Use the remaining frosting to cover the outside of the cake, smoothing it for a clean finish.
Decorate as desired, perhaps with fresh strawberries or chocolate shavings.
Chill the finished cake for at least one hour before serving.