Cream the butter using a rubber spatula. Add the sugar in 2 to 3 parts and mix. Add the milk a little at a time and mix.
Combine and sift the purple sweet potato powder and the flour. Fold them into the mixture from step 1.
Put the dough in a pastry bag.
Line a baking sheet with kitchen parchment paper, and pipe out the cookie dough on it.
Decorate with heart-shaped sugar decorations if you have them.
Bake in a preheated oven at 160 ℃ for 12 minutes.
Story Behind this Recipe
I wanted to make a cute purple cookies decorated with heart-shaped sugar decorations. Apparently purple sweet potatoes turn green when combined with egg, so to keep the beautiful color I didn't use any eggs. However I also heard that if you add a bit of lemon juice to the purple yam it neutralizes the pH so you can add eggs.
When you first add the milk the butter will split, but if you keep mixing it it will come together. You can use maraschino cherries or almond slices instead of the sugar hearts. You can also use cocoa powder or green tea in place of the purple sweet potato powder. You can also substitute margarine for the butter and heavy cream for the milk.