Preheat oven to 350 degrees F (175 degrees C). Grease 12 tartlet pans.
In a medium bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla.
Combine the cocoa, flour and baking powder; stir into the creamed mixture.
Divide dough into 12 tartlet pans. Bake for 15 to 20 minutes in preheated oven. Cool completely before filling.
To make filling: Melt chocolate chips with butter in a double boiler or microwave. Stir in cream and mix until smooth. Fill cooled tartlets with chocolate mixture.