Preheat the oven to 350°F (177°C). Line a 9-inch square baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out of the pan. Set aside.

In a large bowl, whisk together the flour, baking powder, and salt.

In a medium bowl, whisk together the melted butter and brown sugar until combined. Whisk in the egg, egg yolk, and vanilla and mix until combined. Pour the wet ingredients into the dry ingredients and whisk to combine. The batter should be thick. Using a spatula, fold in the chopped candy bars. Spread the batter evenly into the prepared pan. Sprinkle the top with flaky sea salt, if using.

Bake until golden brown, about 30–34 minutes, or until a toothpick inserted in the center comes out mostly clean with only a few moist crumbs. Avoid over-baking.

Cool completely in the pan set on a cooling rack, about 1 hour. Once cool, lift the blondies out of the pan using the parchment overhang on the sides, transfer to a cutting board, and cut into squares.

Cover and store leftover bars at room temperature for up to 1 week.