Preheat the oven to 350°F. Grease an 8-inch square baking pan and line with parchment paper so it extends over 2 sides.

Whisk together thoroughly in a medium bowl:

Melt in a large heavy saucepan over medium heat:

Let the butter brown, stirring occasionally, until light golden brown and fragrant, about 4 minutes. Remove from the heat and stir in until well blended:

Let cool to barely warm. Stir in until well combined:

Stir the flour mixture into the wet ingredients. If desired, stir in:

Scrape into the pan. Bake until the top is golden brown and a toothpick inserted in the center comes out clean, 25 to 30 minutes. Let cool completely in the pan on a rack before cutting.