Although their color is actually less “blond” and more tawny, their vibrant “coffee-intense” taste and incredibly moist texture makes these squares better than good—they’re amazing!
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Line a 9 x 13 inch baking pan with nonstick aluminum foil, dull side up (with a two-inch overhang on each short side) and brush the foil with melted butter. Preheat the oven to 350F. If using a convection mode reduce the oven temperature to 325F (same if using a glass baking pan).
Heat 1 ½ sticks of butter in a medium saucepan and, when almost fully melted, add the brown sugar. Heat the butter, sugar and corn syrup together and combine with a whisk. When homogenous, remove from the stove and stir in the instant espresso and vanilla. Let cool until just warm. Meanwhile, whisk the flour with the baking powder and salt. Sift this into another bowl. Beat the eggs lightly in another bowl and, when the sugar mixture is just warm, stir in the eggs—then add the flour mixture and combine, using a batter whisk. Fold in the chocolate and nuts.
Pour the batter into the prepared pan and spread it out evenly. Place the pan into the preheated oven and bake for 20 to 22 minutes, or until lightly golden brown and the top feels “just set” but still soft (avoid over-baking). Remove from the oven and let the blondie-cake cool on a wire rack—then lift the cake out of the pan and use a sharp knife to cut the slab into small squares.