1/2 cup unsweetened cocoa powder, preferably Dutch process
1/2 teaspoon baking soda
1/4 teaspoon salt
4 ounces unsweetened chocolate, coarsely chopped
1 stick (4 ounces) unsalted butter, cut into tablespoons
1 1/2 cups granulated sugar
1 teaspoon pure vanilla extract
2 large eggs, lightly beaten
4 ounces milk chocolate, chopped into 1/4-inch chunks
1/2 cup chunky or smooth all-natural peanut butter
1/2 cup confectioners' sugar
Preheat the oven to 325°. Line several baking sheets with parchment paper. In a medium bowl, whisk the flour with the cocoa powder, baking soda and salt.
In a medium saucepan, melt the unsweetened chocolate with the butter over low heat. Scrape the melted chocolate into a medium bowl and let cool. Stir in the granulated sugar and vanilla until blended, then mix in the eggs. Stir in the dry ingredients until a soft dough forms, then fold in the milk chocolate.
Roll slightly rounded teaspoons of the dough into 3/4-inch balls. Arrange the balls 2 inches apart on the prepared baking sheets and flatten them slightly. Bake the cookies for about 13 minutes, or until firm outside but still soft in the center; rotate the baking sheets halfway through. Slide the parchment paper with the cookies onto wire racks to cool.
In a small bowl, blend the peanut butter with the confectioners' sugar. Spread the flat side of half the cookies with the peanut butter filling, top with the remaining cookies and serve.
The cookies can be prepared through Step 3 and stored, layered between sheets of wax paper in an airtight container, for up to 1 week.