Combining the best of both worlds - peanut butter and brownies - these sandwich cookies are a favorite of all who try them. Great make ahead option and holiday cookie.
Heat oven to 350 degrees; melt the chocolate chips with the butter until semi-smooth with some lumps of chocolate remaining (it's okay if it's totally melted and smooth, but leaving some of the chips unmelted creates little bursts of chocolate in the finished cookie) - either in a saucepan or microwave. Let cool.
Whisk the eggs, sugar, and vanilla together in a stand mixer for 3-5 minutes. Add the flour and baking powder along with the cooled chocolate, stirring until completely mixed. Let stand for 10 minutes.
Line two cookie sheets with silicone or parchment. Using a small cookie scoop (or just under a tablespoon), drop the dough onto the sheets. Flatten a bit with your hand - they won't spread much, so you can space them fairly close.
Bake 7-8 minutes only - the tops will look cracked like brownies. Cool for 10 minutes on the trays before cooling completely on a wire rack.
Make the Frosting:
Place all of the ingredients in the bowl of a stand mixer and beat until light and fluffy. If the frosting seems too stiff, add more milk or cream - but only 1/2 tsp. at a time.
To Assemble:
Match up pairs of cookies roughly the same size and spread a couple of teaspoons of frosting on one half before topping with the other half.
Notes
To Store: Keep in an airtight container in the refrigerator for a few days or make ahead and freeze for longer storage, about 3 months.