Preheat the oven to 350°F. Line an 8 by 8-inch metal pan* with foil or parchment paper. Spray with nonstick cooking spray.
In a large microwave-safe bowl, heat the butter in 30-second increments until melted. Add the sugar to the hot butter and whisk to combine. Let cool until just warm.
Add the egg, egg yolk, and vanilla and whisk vigorously until smooth. Stir in the salt, baking powder, and flour with a rubber spatula. Gently stir in 1 1/2 cups (255 grams) of the mix-ins.
Evenly spread the batter into the prepared pan. Top with remaining mix-ins.
For an ooey-gooey center, bake until the edges are lightly golden but the center still jiggles slightly, about 25 minutes. For a firmer center, bake until the edges are golden brown and the center is lightly golden and set, 30-35 minutes. Set pan on wire rack and allow to cool before cutting. Sprinkle with flaky sea salt before serving.
Store for up to 3 days at room temperature or up to a week in the fridge. If storing for more than a day, wait to slice into bars until you’re ready to serve to prevent stale edges.
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Notes
*I highly recommend using a metal pan for this recipe. Glass or ceramic will take longer to bake and may cause the edges to overbake before the center is cooked. If you must use glass or ceramic, drop the baking temperature by about 15°F and increase the baking time by 5-10 minutes. If the bottoms of your baked blondies are slightly greasy after removing them from the pan, place them on a wire rack overnight covered in plastic wrap. This overnight sit will allow them to set completely.