Step 1

Preheat oven to 350°F. Grease (with butter) and flour 1 (8-inch) round cake pan. Line bottom of pan with parchment paper; lightly grease parchment paper with butter. Grease (with butter) and flour 1 (8-inch) square cake pan; line bottom of pan with parchment paper; lightly grease parchment paper with butter. Set aside.

Step 2

Place chocolate chips in a small heatproof bowl; pour boiling water over chocolate. Whisk until chocolate is melted and smooth. Let stand until cooled to room temperature, about 15 minutes.

Step 3

Beat butter with a stand mixer fitted with a paddle attachment on medium speed until creamy, about 1 minute. Gradually add granulated sugar, beating until light and fluffy, 3 to 4 minutes. Add egg yolks, 1 at a time, beating just until blended after each addition. Add vanilla and cooled melted chocolate, beating until blended. Whisk together flour, cocoa, baking soda, and salt in a separate bowl. Add flour mixture to butter mixture alternately with buttermilk, beginning and ending with flour mixture, beating on low speed until just blended after each addition.

Step 4

Beat egg whites in a separate bowl with an electric mixer on medium speed until soft peaks form, about 1 1/2 minutes. Gently fold egg whites into batter. Pour batter evenly into prepared pans.

Step 5

Bake in preheated oven until a wooden pick inserted in centers of cake layers comes out clean, about 25 minutes. Cool in pans on wire racks 10 minutes. Remove cake layers from pans; remove parchment paper, and let cool completely on wire racks, about 1 hour.

Step 6

Beat butter with a stand mixer fitted with a paddle attachment on medium speed until creamy, about 1 minute. Gradually add powdered sugar, vanilla, salt, and strawberries, beating on medium speed until mixture is fluffy, about 2 minutes. Beat in desired amount of soft pink food coloring gel until desired color is reached.

Step 7

Place the square layer on a serving platter in a diamond position. Cut round cake in half crosswise, creating 2 perfect half-moon shapes. Position halves against the 2 top angles of the square, forming a heart shape. Spread Frosting over top and sides of cake, and sprinkle with sanding sugar, if desired. Arrange strawberry slices around top edge of cake