In a saucepan, combine raspberries, sugar, and lemon juice. Cook over medium heat until raspberries break down and mixture thickens, about 10 minutes.

Remove from heat and let cool.

Preheat the oven to 350°F (175°C). Grease the baking pan.

In a mixing bowl, whisk together melted butter and sugar until smooth.

Add eggs and vanilla extract, mixing until well combined.

Sift in cocoa powder, flour, salt, and baking powder. Stir until just combined.

Pour half of the brownie batter into the prepared baking pan.

Drop spoonfuls of raspberry sauce over the batter, then pour the remaining brownie batter on top.

Use a knife to swirl the raspberry sauce into the brownie batter.

Bake for 25-30 minutes or until a toothpick inserted comes out with a few moist crumbs.

Let cool in the pan before cutting into squares.