Warm Chocolate-Chunk Blondie Bars
2/3 cup (1 1/3 sticks, 5 1/4 ounces) unsalted butter, melted
1 pound (2 cups, firmly packed) brown sugar, light or dark
1/4 cup (1 3/4 ounces) granulated sugar
1/2 teaspoon salt
1 tablespoon (1/2 ounce) vinegar, white or cider
3 large eggs
1 teaspoon espresso powder (optional)
2 teaspoons vanilla
2 1/2 teaspoons baking powder
2 3/4 cups (11 1/2 ounces) King Arthur Unbleached All-Purpose Flour or King Arthur White Whole Wheat Flour
1 3/4 cups (8 ounces) chocolate chunks or 2 cups (12 ounces) chocolate chips
2 cups (about 8 ounces) coarsely chopped nuts (pecans, walnuts, or almonds)
Preheat the oven to 350 F. Lightly grease a 10" x 15" jelly roll pan or 9" x 13" pan.
Combine the butter, sugars, salt, and vinegar, stirring to combine. If the mixture is super-hot from the melted butter, let it cool to lukewarm. Add the eggs, espresso powder, vanilla, and baking powder, beating well. Stir in the flour, mixing thoroughly, then the chocolate chunks and nuts.
Scoop the batter into the prepared pan, using your wet fingers to smooth the top. Bake the bars for 30 minutes (for the jelly roll pan), or about 38 minutes (for the 9" x 13" pan), until the top is golden and shiny, and the center is just barely baked through. Use the sharp tip of a knife to peek into the center; it should be very damp, but not unbaked. Yes, this puts a hole right in the center of the pan of bars; but you're going to cut them into squares anyway, and just assume the one with the hole will be yours�baker's privilege!
Remove the bars from the oven, and cool to lukewarm before cutting. A plastic knife will glide right through these sticky bars without tearing them. Serve with ice cream, if desired. To serve bars warm after they've cooled, reheat briefly in the microwave, or slip them into a toaster bag and pop into the toaster. Yield: 24 bars from the 9" x 13" pan, 35 bars from the 10" x 15" pan.
Note: Using a jelly roll pan will make thinner bars (about 3/4" thick) than the 9" x 13" pan (a generous 1" thick). But you'll get more bars from the jelly roll pan, a consideration if you're baking for a potluck or bake sale.