Room temperature egg whites into a very clean bowl.
Using an electric mixer, whisk egg whites. Once they begin to foam add the cream of tartar and continue to beat until stiff peaks form.
Add the food coloring if desired and mix in.
Begin folding in the dry ingredients.
Be careful to add the dry ingredients in several batches and fold gently.
The final mixture should look like this. spoon into a piping bag with a medium round attachment and you're ready to start piping.
I printed out some hearts to use as a guide, you can grab the pattern I used below. Place some parchment paper on top then begin to pipe. Start at the top of the heart, swipe downward then do the other side. You might have to smooth out some rough points afterward with a toothpick or small spatula. Allow to rest for 20 to 30 minutes before placing in oven.
Bake at 300F for 8 minutes then lower temp to 290F and bake for another 8-12 minutes.
For the Buttercream Filling:
Cream the room temperature butter.
Sift the sugar and add to the butter.
Beat in the sugar and add the milk or cream until the desired consistency is reached.
Add the food coloring if desired and mix in.
I just used a sandwich bag with a hole cut in the corner to pipe the buttercream onto the hearts. Then add the top and gently press together.