Preheat your oven to 350°F (175°C) and prepare your heart-shaped cake pans by greasing and flouring them or lining them with parchment paper.

In a medium bowl, whisk together the all-purpose flour, cornstarch, baking powder, baking soda, and fine sea salt until well combined.

In a large mixing bowl, use an electric mixer to beat the softened unsalted butter on medium speed until creamy, about 2 minutes. Gradually add the granulated sugar, beating until light and fluffy for 3-4 minutes.

Add the large eggs one at a time, mixing well after each addition. Pour in the pure vanilla extract and mix until fully incorporated.

In a separate container, whisk the whole milk and sour cream until smooth. Gradually add the dry mixture to the butter mixture in three parts, alternating with the milk mixture.

Reserve 2 cups of the plain batter. Divide 1 cup and tint with golden yellow gel food coloring, another cup with medium brown, and ½ cup with black gel food coloring.

Pour half of the plain batter into a lined 9x13 inch pan, pipe blobs of brown and black batter, and cover with yellow batter. Bake for 12-16 minutes.

Pour a layer of plain batter into the prepared heart-shaped pans. Dollop the colored batters into the plain batter, swirling slightly. Bake for 25-32 minutes for 8-inch pans or 40-50 minutes for a deep 9-inch pan.

Allow cakes to cool in the pans for 15 minutes, then transfer to a wire rack to cool completely. Level the tops of the cakes for even stacking.

Beat the softened unsalted butter until smooth, gradually adding sifted powdered sugar and heavy cream until the frosting is light and fluffy.

Spread reserved frosting between the cake layers, then pipe colored patterns on the sides and top of the cake for decoration.

Chill the decorated cake for about 30 minutes before slicing. Use a warm knife for clean cuts.