Preheat the oven to 350°F (175°C). Grease an 8x8-inch baking dish or line it with parchment paper for easy removal.

In a medium bowl, combine the flour, granulated sugar, and salt. Stir in the melted butter, egg, and vanilla extract until well combined.

Pour the mixture into the prepared baking dish and spread it evenly.

In another bowl, mash the raspberries with a fork until slightly chunky. Stir in the lemon juice and lemon zest.

Drop spoonfuls of the raspberry mixture over the batter in the baking dish. Use a knife or toothpick to gently swirl the raspberry mixture into the batter, creating a marbled effect.

Bake for 25-30 minutes, or until the edges are golden and a toothpick inserted in the center comes out clean.

Let the bars cool in the pan for about 10 minutes, then lift them out using the parchment paper (if used) and allow them to cool completely on a wire rack.

Once cooled, dust the top with powdered sugar before cutting into squares.