Preheat oven to 350 °F (175 °C). Line an 8×8 inch (20 × 20 cm) baking dish with parchment paper.

In a mixing bowl, stir together melted butter and brown sugar until well combined.

Add beaten egg and vanilla extract; mix until smooth.

Add flour and a pinch of salt; stir just until combined and no dry streaks remain. The batter should be dense and fudgy.

Spread the batter evenly in the prepared pan.

In a small bowl, mix granulated sugar with cinnamon. Sprinkle evenly over the top of the batter.

Bake for about 22 minutes, or until edges begin to pull away from the pan and a toothpick near the center comes out mostly clean.

Cool completely in the pan (about 30 minutes) before lifting out and slicing into 16 squares.

Store in an airtight container; best kept at room temperature up to 3 days or refrigerated for 4–5 days.