I had posted a link to the recipe, but the author seems to have removed it so I can’t credit the author.

3/4 cups butter softened
1 1/2 cups granulated sugar
3 eggs
1/2 tbsp vanilla paste or regular vanilla extract
1/2 tsp baking soda
2 tsp Baking powder
1/4 tsp salt (omit if using salted butter)
2 1/2 cups flour
1 1/3 cups milk

Cream sugar and butter together in a large bowl. Beat eggs in one at a time. Add vanilla, soda, baking powder and salt and beat together.
Add flour and milk alternately a little at a time until well combined.
Batter should be nice and fluffy.
Remove about a 1/3 of the batter (about 1 1/2 cups) and place in small bowl Add red food colour to the batter to get a nice shade of pink. Cover plain batter and set aside (refrigerate if desired)
Put pink batter in an 8×8″ baking pan and bake at 350F for about 20 minutes until a toothpick inserted in the center comes out clean.
Remove from pan and allow to cool completely.
Cut out heart shapes from the pink cooled cake with a cookie cutter.
Place 2 tbsp of the plain batter in the bottom of a cupcake liner in a cupcake pan. Place one pink cake heart pointy side down in the center of the batter making sure to push it all the way down to the bottom. Cover the top of the heart with about 1 tbsp of additional plain batter.
Bake the cupcakes at 350 F for about 20 minutes until a tooth pick inserted in the plain batter comes out clean.
The hearts inside won’t burn or overcook.
Make sure you note which way the hearts are facing as cutting them the wrong way will result in a pink blob instead of a heart.

I used a very light and fluffy cooked frosting. One of the best home-made frostings I have ever made. I will post the recipe tomorrow.
Thanks for watching

I don’t I explained myself very well about the number of cupcakes this makes. There is enough for about 18 or so hearts from the red cake. The white batter that is left will make about 15-16 cupcakes with hearts in them.

Frosting I used on the cupcakes:
http://www.youtube.com/watch?v=WEefX3sOWFk