4tablespoonsunsalted butter or dairy-free buttery sticks
Make the cookies: Preheat the oven to 350° and line 2 baking sheets with parchment paper. In a large saucepan, combine two-thirds of the chocolate with the butter and melt over low heat, stirring, until smooth. Let cool.
In the bowl of a standing electric mixer fitted with a paddle, beat the eggs with the brown sugar, granulated sugar and vanilla at high speed until thickened, about 5 minutes. Reduce the speed to low, and mix in the melted chocolate followed by the dry ingredients. Add the remaining 4 ounces of chopped chocolate. Refrigerate the dough just until it’s firm enough to scoop, about 10 minutes.
Scoop the batter into 5 mounds on each baking sheet and bake in the upper and lower thirds of the oven for 15 to 20 minutes, until the tops are crackled and shiny and the cookies are just barely set; shift the pans from top to bottom and front to back halfway through. Let the cookies cool for 5 minutes on the pans, then slide the parchment onto wire racks to let them cool completely.
Make the frosting: In a saucepan, melt the chocolate with the butter over low heat, stirring until smooth. Invert the cookies and spread each with a heaping spoonful of the frosting, spreading it to the edges. Let cool until set before serving. The cookies can be refrigerated in an airtight container for up to 3 days.