Marble Blondies

Preheat the oven to 350°F with a rack set in the middle. Lightly spray a 9-inch square baking pan with nonstick cooking spray and line with a sheet of parchment that completely covers the bottom with a slight overhang on two sides. Lightly spray the pan once more, greasing the parchment and sides.

In a large bowl, whisk the melted butter, brown sugar, granulated sugar, 2 eggs, and vanilla until well combined with no lumps from the brown sugar, lightened in color, and glossy and creamy looking, about 2 minutes.

In another large bowl (it will be used again later), whisk the flour, salt, and baking powder to combine. Use a rubber spatula to fold the flour mixture into the butter mixture until just combined with no dry streaks remaining.

Transfer 1/3 of the blondie batter (about 1 rounded cup) back to the empty bowl that had the flour mixture. Add the melted chocolate, cocoa powder, espresso powder (if using), and remaining egg yolk and fold together until just combined.

Using two separate rubber spatulas so as to not cross batters, fold the white chocolate chips into the blondie batter and the semi-sweet chocolate chips into the brownie batter until evenly distributed.

In the prepared square baking pan, marble the batters. For the first layer of batter, use about half of the blondie batter and half the brownie batter to drop alternating rounded tablespoons of the blondie batter and level tablespoons of the brownie batter across the bottom of the pan. Use a small offset spatula or the back of a spoon to gently spread the blondie batter and gently tap the brownie batter with your fingertips to create as even a layer as possible. The layer of batter should loosely resemble a checkerboard. For the second layer, using the rest of each batter, drop rounded tablespoons of the blondie batter directly on top of the chocolate batter in the pan and level tablespoons of the brownie batter directly on top of the blondie batter in the pan. Repeat leveling out the batters with an offset spatula/spoon and your fingertips.

Using the side of a small offset spatula or the spine of a butter knife or a thick skewer, marble the batters. Carefully and thoughtfully use quick motions to drag and swirl the top layer of batters into one another. Take care not to swirl too much to the point of being too blended as the batters do bake into each other slightly and swirling too much can muddy up their appearance.

Bake the blondies on the middle rack of the oven until risen, set, and a toothpick comes out with clean or with moist crumbs (not wet batter), 30 to 40 minutes.

Let the blondies cool slightly in their pan set over a wire rack for 15 minutes. Lift the blondies out of their pan using the parchment overhang and transfer to the wire rack to finish cooling. Cool the blondies completely before slicing, at least 1 hour. Use a sharp knife to slice the blondies into 16 squares and serve.