Preheat oven to 350. Put mini cupcake liners into mini cupcake pan.
Prepare brownie mix according to package directions then fold in mini chocolate chips. Spoon batter into the mini cupcake paper lined pan till each cup is about 3/4 full. Keep in mind that it makes about 40 or more brownie bites so if you only have one pan you'll have a couple bakes. Bake for 15 to 18 minutes. Put baked brownies on cooling rack and cool completely before icing them.
While your brownies are baking prepare icing. Put soft butter in mixer and whip with flat beater. Do not use a whip on your icing that only adds air to your icing than it does not go on as well. While butter is whipping melt chocolate chips in microwave or over a double boiler. Now add powdered sugar to the butter it will be rather thick, so now you can use milk to thin a bit but not too much. Add melted chocolate and vanilla than beat on medium high speed (about 7 on a kitchenaid) for 8 to 10 minutes. Beating the icing for a longer time makes a smooth and creamy icing. I'm a baker and cake decorator, the biggest mistake make when making icing is it is now beat long enough using the correct beater. Set icing aside till all brownies are baked and cooled.
When brownies are completely cooled ice them. You can either use a spatula or a pastry bag and star tip. Now all you need is a cup of coffee or glass of milk for a nice afternoon snack. Simple to make yummy to eat! Store in air tight container.