My Buttery Blondies Will Make You Quit Brownies Forever

Cut 1 stick unsalted butter into 8 pieces and place in a large microwave-safe bowl. Microwave until melted, about 70 seconds. (Alternatively, melt the butter on the stovetop and transfer to a large bowl.) Add 1 cup packed light brown sugar and 1/4 teaspoon kosher salt, and whisk until combined. Add 1 large egg and 1 teaspoon vanilla extract, and whisk until smooth and shiny.