Using an electric mixer, mix the peanut butter, milk, and sugar for 2 minutes at high speed. Add the egg and mix well for another minute.
Mix the flour and baking powder and add to the previous mixture. Mix until homogeneous.
Using your fingers or an ice-cream scoop, form small balls and slightly flatten the top with a fork. Place on a cookie sheet covered with parchment paper or a silicone mat.
Bake for about 10 minutes or until lightly browned.