Prepare a flat shallow pan by greasing it and preheating the oven.
Cream 1/2 a cup of margarine and 1/2 a cup of sugar together till light and fluffy. Add in one egg and mix it in. Add the lemon juice to 1/2 a cup of milk before adding it into the mixture. Add in 1/2 a teaspoon of vanilla essence.

Mix together 1 cup of flour, the cocoa powder and food color. Sieve the dry ingredients into the wet mixture and fold in until well incorporated. Place the batter into the prepared pan and bake at 170 degrees Celcius for about 15-20 minutes or until a skewer placed in the middle comes out clean. Leave it to cool completely/freeze to make cutting out easier.

Meanwhile, Prepare a bread tin. Prepare the vanilla batter by Creaming 1 cup of margarine and 1 cup of sugar together till light and fluffy. Add in the 2 eggs,remaining vanilla essence, and 1 cup of milk and mix them in well. Sieve the 2 cups flour into the wet mixture and fold in until well incorporated.

Cut out heart shapes from the red velvet slab cake.

Here are the shapes

Scoop into the cake pan about 4 tablespoons of batter and flatten.

Pack in the cut outs tightly and fill in the rest of the vanilla cake batter.

Bake for about 30 minutes at 170 degrees celcius or until a skewer inserted on the sides comes out clean.
Let it cool before cutting it to reveal the shape.