These brownies are named for their dark, fudge-like appearance and texture. Ironically, they look much sweeter than they taste. The addition of light corn syrup in the batter creates a wonderful shine on the top of the baked brownies and, although most brownies have a tendency to crack or sink in the center, these have never done either. As for the peanut butter filling–it elevates the entire brownie eating experience to new heights! But be forewarned; These brownies disappear immediately in my house. When younger, I practically had to frisk my son in the morning. Once I found wrapped brownies in each pocket of his pants–on the sides, in the back and down the legs! For best flavor, if your instant coffee is granular (not powdered), pulverize it to a fine powder using an electric spice grinder; or put the granules in a plastic bag and crush them with a rolling pin or a wine bottle. Also, if you’re using the peanut butter filling, use the optional chocolate chips in the batter instead of walnuts or peanut butter chips.
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In a large bowl, thoroughly combine peanut butter with confectioner’s sugar. Knead briefly with your hands until smooth. Lightly dust a 14-inch sheet of waxed paper with confectioner’s sugar and place peanut butter mixture on the center of the paper. Lightly sprinkle the top of the filling with more confectioner’s sugar and place another sheet of waxed paper on top. Press filling to flatten gently and, using a rolling pin, roll out filling 1/4 inch thick into a 9×13-inch rectangle. Refrigerate until needed.
If using a glass baking dish, preheat the oven to 325o F; If using a metal pan, preheat to 350o F. Line baking dish with aluminum foil (dull side down for glass, dull side up for metal), allowing a 2-inch overhang at each end. Brush foil with melted butter and set aside.
In a 2-quart heavy-bottomed saucepan, melt the 2 sticks butter with chocolate over low heat. Remove from heat and stir in instant coffee and vanilla; let cool slightly. In a medium-sized mixing bowl, whisk together both flours, baking powder and salt. Sift this mixture into another bowl. Whisk sugar and corn syrup into the melted chocolate mixture until smooth. Add eggs (1 at a time) and egg yolk, whisking well after each addition. Add dry ingredients to pan with chocolate mixture and, using the whisk, gently but thoroughly combine until smooth. If desired, fold in walnuts or chips, using a rubber spatula.
If using peanut butter filling, pour 1/2 the brownie batter into the prepared baking dish and gently spread to each corner, using a rubber spatula. Peel off 1 sheet of waxed paper from filling and invert so filling sits directly on top of batter. Peel off the remaining piece of waxed paper and pour on remaining batter, spreading to enclose filling. If not using the filling, simply pour all of the batter into the prepared pan.
Bake in preheated oven until a toothpick comes out almost clean when inserted in the center, 23 minutes (avoid over baking). Place on a wire rack to cool thoroughly. Once cool, cover pan with aluminum foil and let sit at least 4 hours–preferably overnight. Then lift the brownie “cake” out of the pan (using the overhang of foil as a handle) and slice into 2-inch squares using a sharp knife. For easier slicing, wipe off any accumulated chocolate that clings to the knife after each cut.
To best preserve moistness, store sliced brownies in an airtight tin separated by sheets of waxed paper or wrap each brownie in pliable plastic wrap. Either way, store at room temperature.
The peanut butter filling can be made 3 days ahead and kept in the refrigerator until needed. However, if the filling is very chilled, remove it from the refrigerator an hour before using so it won’t affect the overall baking time of the brownie mixture.
As a time saver when assembling, sift the dry ingredients days ahead and leave in a covered bowl at room temperature. Give the mixture a good swish with the whisk before incorporating it into your batter.