Step 1: Prepare the Pan

1. Preheat your oven to 350°F (175°C) and position a rack in the middle.
2. Lightly spray a 9-inch square baking pan with nonstick cooking spray.
3. Line the pan with parchment paper, leaving an overhang on two sides for easy removal. Spray the parchment lightly.

Step 2: Make the Blondie Batter

1. In a large bowl, whisk together the melted butter, light brown sugar, and granulated sugar until smooth and glossy (about 2 minutes).
2. Add the eggs and vanilla extract, whisking until fully combined.
3. In a separate bowl, whisk together the flour, salt, and baking powder.
4. Gradually fold the dry ingredients into the wet mixture using a rubber spatula until just combined.
5. Gently fold in the white chocolate chips.

Step 3: Make the Brownie Batter

1. Transfer ⅓ of the blondie batter (about 1 cup) to a separate bowl.
2. Add the melted chocolate, cocoa powder, espresso powder (if using), and egg yolk. Mix until fully incorporated.
3. Stir in the semi-sweet chocolate chips.

Step 4: Layer and Marble the Batters

1. Drop alternating tablespoons of blondie and brownie batter across the bottom of the prepared pan.
2. Use an offset spatula or the back of a spoon to gently spread the batter into an even layer.
3. Repeat the process with the remaining batter, alternating layers.
4. Drag a butter knife or skewer through the batter in swirling motions to create a marbled effect.

Step 5: Bake the Blondies

1. Bake for 30–40 minutes, or until a toothpick inserted in the center comes out with moist crumbs (not wet batter).
2. Let the blondies cool in the pan on a wire rack for 15 minutes.

Step 6: Cool and Slice

1. Using the parchment paper overhang, carefully lift the blondies out of the pan.
2. Allow them to cool completely on a wire rack for at least 1 hour.
3. Slice into 16 squares with a sharp knife.

Step 7: Serve and Store

1. Serve immediately or store in an airtight container at room temperature for up to 4 days.