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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 ½ cups (255 grams) mix-ins.
Pour the batter into the prepared pan. Top evenly 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.