1. First, brown the butter. Place it in a medium saucepan over low heat to cook. Check it after about 10 minutes; it should be a medium-brown color, not too light, not too burned. Once it reaches that color, take it off the heat and let it cool.
2. Preheat the oven to 350F and grease a 9 x 13 - inch pan.
3. In a bowl of a stand mixer fitted with paddle attachment, beat the cooled brown butter and brown sugar until creamy. Add the eggs and vanilla and mix, scraping the sides of the bowl. Add the flour, baking powder, and salt and mix until combined.
4. Press the mixture evenly into the prepared pan.
5. Bake the blondies for about 35 minutes, or until a toothpick inserted into the center comes out with only a few crumbs stuck to it. (I like these blondies gooey, so I go with a little less time). Let cool completely on a wire rack, then cut them and wrap them individually in plastic wrap so the sides don’t dry out. Store in the freezer or at room temperature.
Options: Add up to ¾ cup of your own ingredients, like pecans, chocolate chips, shredded coconut, butterscotch chips, peanut butter chips, or chopped dry cherries and white chocolate chips. Anything goes!