Heat oven to 350°F. Grease and flour 8-inch square baking pan.
Stir together flour, granulated sugar, brown sugar, cocoa, baking soda and salt in medium bowl. Add water, oil, vinegar and vanilla; beat with whisk or spoon until smooth. Pour batter into prepared pan.
Bake 18 to 20 minutes or until wooden pick inserted in center comes out clean. Cool 10 minutes; remove from pan to wire rack. Cool completely.
Transfer to cutting board. Using 3-1/4 inch heart-shaped cookie cutter, cut cake into 4 hearts. Prepare CHOCOLATE FROSTING; spread over tops of two hearts. Place remaining two hearts on top. Garnish as desired. 2 small cakes.
CHOCOLATE FROSTING: Place butter in small microwave-safe bowl. Microwave at MEDIUM (50%) 20 seconds or until butter is melted. Stir together powdered sugar and cocoa; add to butter mixture alternately with milk, beating with spoon or whisk until smooth. Stir in vanilla. About 1/3 cup frosting.