Preheat the oven to 350°F (175°C) and grease or line an 8x8-inch baking pan with parchment.

In a mixing bowl, whisk the melted butter and brown sugar until smooth and glossy. Let the butter cool for a minute.

Add the egg and vanilla extract to the butter-sugar mixture. Stir until fully incorporated and slightly fluffy.

Stir in the lemon juice and lemon zest until evenly distributed.

In a separate bowl, whisk together the flour, baking powder, and salt.

Gently fold the dry ingredients into the wet mixture until just combined.

Pour the batter into the prepared pan and smooth the top with a spatula.

Bake for 20-25 minutes, rotating the pan halfway through. The edges should be lightly golden; a toothpick inserted into the center should come out with a few moist crumbs.

Cool completely in the pan on a wire rack before lifting out and cutting into squares.