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Preheat the oven to 325 degrees. Line a 13- by 9-inch glass baking pan with foil, leaving an overhang around the edges. Spray foil generously with cooking spray and lightly dust with flour. Set pan aside.
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Combine the butter, 2 2/3 cups (16 ounces) of the chocolate chips, and the 6 ounces unsweetened chocolate in the top of a double boiler set over simmering water. Heat until butter and chocolate are melted and mixture is smooth. Remove from the heat and allow to cool slightly so you don’t scramble your eggs.
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Whisk together the eggs, espresso powder, vanilla, and sugar in a large mixing bowl. Add the chocolate mixture a couple of tablespoons at a time, whisking after each addition; once all of the chocolate mixture is incorporated; set aside to cool to room temperature.
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Sift together 1 cup of the flour, the baking powder, and the salt in a small mixing bowl. Stir into the cooled chocolate-egg mixture until just combined.
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Combine the remaining 2 cups of chocolate chips with the remaining 1/4 cup flour in a small mixing bowl. Fold into the chocolate-egg mixture.
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Pour half of the batter into the prepared pan and smooth the top. Arrange the chopped Snickers in a single layer over the batter. Top with the remaining batter and smooth with a spatula. Bake for 35 minutes—the brownies will not be firm but don’t bake them longer. Allow to cool to room temperature, then refrigerate overnight.