Line a 9-inch heart-shaped nonstick cake pan with a heart-shaped piece of parchment paper cut to fit inside bottom of pan.
Preheat oven to 350°F. Whisk together flour, cocoa powder, baking soda and salt in a medium bowl. Combine buttermilk and vanilla extract in a separate bowl or liquid measuring cup. Beat butter and sugar in the bowl of an electric mixer fitted with the paddle attachment about 3 minutes or until light and fluffy. Reduce speed to low and add eggs, whole egg first then the yolk, beating well after each addition. Alternately add flour mixture and buttermilk mixture in several additions, starting and ending with flour mixture just until combined.
Spread batter in prepared cake pan and bake about 45 minutes or until cake tester inserted into center of cake comes out clean. Cool in pan on wire rack 10 minutes. Invert to cool completely. Once cool, using a serrated knife, cut cake crosswise in half.
Whisk together confectioners’ sugar and cocoa powder in a medium bowl. Beat butter in an electric mixer on medium-high with paddle attachment until smooth. Reduce speed to low and gradually add confectioners’ sugar mixture until smooth. Add milk and vanilla extract and beat until smooth. Change to whisk attachment and beat 1 minute or until fluffy.
Spread 1 cup frosting between layers and cover cake with remaining frosting. To decorate with the raspberries first outline the bottom of cake with the raspberries. Then outline the top edge of the cake with raspberries. Fill in the sides and the top with the remaining raspberries until the entire cake is covered.