Preheat the oven to 350 degrees F. Line a small loaf pan with parchment paper.
Melt the butter in a small saucepan over low heat. Remove from heat and stir in the 1/2 cup sugar until combined.
Whisk in the egg and vanilla extract until smooth.
Stir in the flour, cocoa powder, and salt until no dry streaks remain.
In a separate small saucepan, combine the raspberries, 1 tablespoon sugar, and lemon juice. Cook over medium heat for 4 minutes, stirring and mashing until the mixture thickens slightly.
Pour the brownie batter into the prepared pan. Drop spoonfuls of the raspberry mixture over the top and drag a knife through to create swirls.
Bake for 22 minutes until the center looks set. Cool completely before slicing.