Preheat the oven to 350°F. Line a 9x13" pan with parchment paper, leaving it hanging slightly over the side (this will make it easy to lift out of the pan to cut) and set aside.

In a large mixing bowl, whisk together the melted, cooled butter and brown sugar until combined. Then whisk in the 2 eggs and the yolk. Next, whisk in the vanilla extract.

Add the flour, baking powder and salt to the bowl. Fold everything together with a rubber spatula. Batter will be thick.

Spread the batter into your prepared baking pan. For prettier blondies, press a few additional of whatever you mixed in, onto the top of the batter.

Bake for 30 minutes, or until a toothpick comes out mostly clean, but with just a few moist crumbs on it (similar to when you make brownies). After the 30 minute mark, if they're not done yet, check every couple of minutes, so they don't overbake.

Once the pan is removed from the oven, place on a wire rack. Allow to cool completely before cutting them into squares.

Store in an airtight container at room temperature for up to 1 week.