8 ounces good-quality milk chocolate (4 ounces coarsely chopped and 4 ounces cut into ¼-inch chunks)
½ cup (1 stick) unsalted butter
1 ½ cups granulated sugar [I prefer soft brown sugar]
2 large eggs
1 teaspoon pure vanilla extract
Preheat oven to 325 degrees F. In a bowl, whisk together flour, cocoa powder, baking soda and salt. Melt 4 ounces of coarsely chopped chocolate with the butter in a small heatproof bowl set over a simmering pan of water; let cool slightly.
Put chocolate mixture, sugar, eggs and vanilla in the bowl of an electric mixer fitted with the paddle attachment. Mix on medium speed until combined. In small amounts, add the flour mixture and blend until combined. Stir in the milk chocolate chunks.
Using a 1 ½ inch ice cream scoop, drop cookies onto a baking sheet approximately 2 inches apart on a cookie sheet lined with parchment paper or use a Silpat. Bake for until the cookies are flat and start to break apart, approximately 15 minutes. Let cool on parchment on a wire rack. Cookies can be stored in an airtight container at room temperature for up to 3 days.