Mix all the ingredients in a bowl; add the milk spoon by spoon until the dough has the right consistency (it shouldn`t be too runny).
For the cookie dough filling:
Mix the flour, baking powder and salt and set aside.
Beat the butter and sugar in a large bowl with the mixer until creamy, then add the egg and vanilla extract and stir again with the mixer. Then stir in the flour mixture until everything is mixed. Then stir in the chocolate chips until they have spread throughout the mass.
Now shape the cookie dough into 24 balls and freeze for about 2-3 hours until they have set.
Completion:
Now divide the muffin batter into 24 muffin cases, which are about 2/3 full. Place a cookie dough ball on each of the muffin batter and press in a little if necessary.