Heat oven to 350°F. Grease two 8-inch round cake pans. Line bottoms of pans with parchment or wax paper; grease and flour. In small bowl, combine cocoa powder, food coloring, water, and vanilla.
In cup, combine milk and lemon juice. In bowl, with electric mixer on high, beat butter and sugar until fluffy, about 3 minutes. Beat in eggs. Reduce mixer speed to low; add flour, baking soda, and salt. Add milk and cocoa mixtures, and beat until combined. Divide batter among pans, and bake 25 minutes, or until a toothpick inserted comes out clean. Cool 10 minutes. Remove from pans and cool completely.
In medium bowl over simmering water, with electric mixer on low, beat together first 5 ingredients until combined. Increase mixer speed to high, and beat 10 to 12 minutes, scraping sides of bowl, until stiff peaks form. In bowl, mix together 1 cup frosting and a tiny drop of food coloring until frosting is pale pink. Transfer to resealable plastic bag. Spoon 1 cup white frosting into another plastic bag.
Spread 1/2 cup of frosting from bowl on one cake layer. Top with remaining cake, and frost sides and top of cake with remaining frosting. Snip off one corner of each frosting bag, and pipe dollops of frosting on top of cake. Decorate with heart-shaped sprinkles.