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

Melt the butter. Melt the butter on the stovetop or in the microwave. Let it cool for 3–5 minutes so it doesn’t scramble the egg.

Mix the wet ingredients. In a medium bowl, whisk the brown sugar and melted butter until glossy and thick. Whisk in the egg and vanilla until smooth.

Add the dry ingredients. Sprinkle the flour and a pinch of salt over the bowl. Fold gently with a spatula until just combined. Stop as soon as no dry streaks remain. The batter will be thick.

Spread and smooth. Scrape the batter into the lined pan. Use the spatula to press it into the corners and smooth the top.

Bake. Bake for 18–22 minutes, or until the edges are set and lightly golden and the center looks just barely underdone. A toothpick should come out with moist crumbs, not wet batter.

Cool and slice. Let the pan cool on a rack for at least 20–30 minutes. Lift the slab out using the parchment and cut into 9–16 squares.