Prep the pan. Heat the oven to 350°F (175°C). Line an 8-inch square pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.

Melt the butter. In a microwave-safe bowl or small saucepan, melt the butter until just liquid. Let it cool 2–3 minutes so it doesn’t scramble the egg.

Whisk the wet ingredients. In a medium bowl, whisk the melted butter, brown sugar, and granulated sugar until smooth and glossy. Add the egg and vanilla. Whisk until the mixture looks thick and shiny.

Combine the dry ingredients. In a separate bowl, stir together the flour, baking powder, baking soda, salt, and cinnamon.

Bring it together. Add the dry ingredients to the wet. Switch to a spatula and fold gently until no dry flour remains. The batter will be thick. Fold in your chosen mix-ins.

Spread and bake. Scoop the batter into the pan and smooth the top. Bake 18–22 minutes, until the edges are set and deep golden, and a toothpick comes out with a few moist crumbs.

Cool, then slice. Let the bars cool in the pan at least 15–20 minutes. Lift out using the parchment and cut into squares. Warm is wonderful.