Preheat your oven to 350°F / 175°C.
Grease an 8-inch square baking pan or line it with foil, leaving an overhang to easily lift the blondies out later, and grease the foil.
In a small saucepan, melt the butter over low heat.
Transfer the melted butter to a mixing bowl.
Using an electric mixer, beat in the brown sugar until the mixture is smooth and glossy.
Beat in the egg and vanilla extract, scraping down the sides of the bowl as needed.
Add the salt, then gently stir in the all-purpose and whole wheat flours until just combined. Be careful not to overmix. If you're using any mix-ins, fold them in now.
Pour the batter into the prepared pan and spread it into an even layer.
Bake for 20 to 25 minutes, or until the edges are golden and the middle is just barely set. If the blondies seem to be baking unevenly, rotate the pan halfway through.
Let the blondies cool completely on a wire rack before lifting them out of the pan and cutting into squares. Store in an airtight container at room temperature for up to 3 days.
Patience is key! Allowing the blondies to cool completely before slicing is the secret to getting clean, perfect squares that don't fall apart.
