1. Preheat the oven to 350°F/177°C. Line an 8x8 square baking pan with parchment paper to hang over the edges (to remove the brownies easily from the pan).

Brownie Batter

1. In a microwave-safe bowl, microwave the butter and chopped chocolate for 30 seconds. Mix together and continue to microwave for 15-second intervals until melted and smooth.
2. In a large bowl, whisk together the sugar and egg for 2 minutes or until light and smooth. Slowly pour in the chocolate mixture and vanilla while whisking to combine.
3. Using a rubber spatula, gently fold in the flour, cocoa powder, cornstarch, and salt until just combined. Do not overmix.

Blondie Batter

1. In a medium bowl, whisk the flour, baking powder, and salt. Set aside.
2. In a large bowl, whisk the melted butter and sugars.
3. Add the egg and vanilla and whisk vigorously for 1 minute until smooth and light in color.
4. Gently fold in the dry ingredients until just combined. Do not overmix!
5. Fold in the chopped chocolate.
6. Spread 3/4 of the batter in an even layer in the prepared baking pan (it's ok if it doesn't cover the entire bottom of the pan).
7. Spread the brownie batter into an even layer over the blondie batter. Scatter teaspoonsful of blondie batter over the brownie and gently swirl them together.
8. Bake for 24 to 27 minutes until the top is set and the sides are starting to pull away from the pan. Allow the brownies to cool completely in the pan before slicing.