Hazelnut Swirl Blondies

Preheat oven to 350°F. Coat a 9-inch square baking pan with cooking spray. Line with parchment paper, leaving a 2-inch overhang on 2 sides. Melt butter in a large pot over medium, stirring occasionally to ensure butter doesn't brown, 3 to 4 minutes. Remove from heat; whisk in sugar until well-combined. Whisk in eggs and vanilla.

Add flour, baking powder, and kosher salt to butter mixture; stir until batter is smooth. Stir in hazelnuts, if using. Transfer batter to baking pan. Spread in an even layer and smooth top.

Dollop chocolate-hazelnut spread in tablespoonfuls on top of blondie batter. Drag a wooden spoon or a skewer through spread to swirl it slightly (it won't be evenly spread out over batter). Sprinkle with flaky sea salt.

Bake until top of blondies is set and edges are golden (center will still be a bit gooey), 22 to 24 minutes. Transfer baking pan to a wire rack and let blondies cool completely, about 2 hours. Using parchment paper overhang as handles, remove blondies from pan and slice.