Heat oven to 350°F. Grease and flour an 8x8-inch baking pan.
In a bowl whisk the flour, sugar, brown sugar, cocoa powder, baking soda and salt. Set aside.
In a small bowl or measuring cup mix together the water, oil, vinegar and vanilla.
Make a well in the center of the flour mix. Pour the liquid over the mix, stir to combine starting at the center and working out toward the sides. Mix until smooth.
Pour batter into prepared pan. Bake for 16-20 minutes or until toothpick comes out with just a few crumbs on it. Cool until pan is no longer hot to the touch, approximately 10 minutes. Flip cake out of the pan and place on wire cooling rack.
While the cake is cooling prepare the frosting: In a bowl combine the cocoa powder and powdered sugar. Stir in the melted butter, mix until smooth. Add vanilla and milk, beat with whisk until smooth. Set aside.
Use a heart-shaped cookie cutter that’s approximately 3-inches, it needs to be small enough to cut out 4 pieces of cake. Place the cooled cake on a cutting surface, using the cookie cutter cut out 4 pieces of cake.
Spread chocolate frosting over the tops of all 4 cake hearts. Designate two hearts as bottoms, top each of the bottoms with one of the remaining hearts, this should give you two 2-layer mini cakes. Decorate as desired. Sprinkles, sugars, have fun with it!