Fudgy Peanut Butter Cup Brownies

12 tablespoons unsalted butter, cut into tablespoons
12 ounces semisweet chocolate, coarsely chopped
6 large eggs (at room temperature)
1 3/4 cups granulated sugar
2 teaspoons vanilla extract
1 1/2 cups all-purpose flour
1/4 teaspoon salt
24 miniature chocolate-covered peanut butter cups

Position a rack in the center of the oven and preheat to 350 degrees F. Line a 13 x 9-inch baking pan with aluminum foil so that the foil extends 2 inches beyond the two long sides of the pan. Lightly butter the bottom and sides of the foil-lined pan. Melt the butter and chocolate together. Cool until tepid.

In a medium bowl, using a wire whisk, beat the eggs until foamy. Add the sugar and beat until blended. Add the cooled chocolate mixture and mix until smooth. Stir in the vanilla extract. Stir in the flour and salt until well combined. Scrape half the batter into the prepared pan and smooth the top with a rubber spatula. Arrange the peanut butter cups evenly over the batter, in four rows of six cups each. Press down lightly on each cup. Pour the remaining batter into the pan, and carefully spread level over the peanut butter cups. Bake the brownies for 25 to 30 minutes, until a wooden pick inserted into the center comes out with a few moist crumbs clinging to it.

Cool the brownies completely in the pan set on a wire rack. Using the two ends of the foil as handles, lift the brownies out of the pan. Cover with plastic wrap and refrigerate for at least four hours or overnight.

Chocolate Sauce
8 ounces milk chocolate, broken into pieces
1/4 cup boiling water
1/2 teaspoon vanilla extract
1/4 cup unsalted roasted peanuts, chopped

Do not make the sauce until the brownies have chilled. Place the milk chocolate in a food processor fitted with the metal chopping blade. Process for 10 to 20 seconds, until finely chopped. Transfer the chocolate to a medium bowl. Pour the boiling water over the chocolate. Let the mixture stand for 30 seconds to melt the chocolate.

Gently whisk until smooth. Stir in the vanilla extract. Remove the plastic wrap from the brownies and invert onto a cutting board or large plate. Carefully peel off the foil. Re-invert the brownies onto a smooth surface and cut evenly into 24 squares, so that each brownie has one peanut butter cup in its center. To serve, place one brownie on each dessert plate, top with some of the milk chocolate sauce, and sprinkle with some of the chopped peanuts.