Preheat the Oven: Preheat your oven to 350°F (175°C). Grease or line an 8x8 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Prepare the Crust: In a mixing bowl, combine the flour, sugar, and salt. Cut in the cold cubed butter using a pastry cutter or fork until the mixture resembles coarse crumbs. Press the mixture evenly into the bottom of the prepared baking pan.
Bake the Crust: Place the crust in the preheated oven and bake for 15-18 minutes, or until lightly golden brown around the edges. Remove from the oven and set aside to cool slightly.
Prepare the Lemon Filling: In a separate mixing bowl, whisk together the sugar, eggs, lemon juice, lemon zest, flour, baking powder, and salt until smooth and well combined. Pour the lemon filling over the partially baked crust, spreading it out evenly.
Add the Raspberry Swirl: Drop spoonfuls of raspberry jam or preserves onto the surface of the lemon filling. Use a knife or toothpick to gently swirl the raspberry jam into the lemon filling, creating a marbled effect.
Bake the Bars: Return the baking pan to the oven and bake for an additional 20-22 minutes, or until the filling is set and the edges are lightly golden brown.
Cool and Serve: Remove the pan from the oven and allow the Raspberry Lemon Bars to cool completely in the pan on a wire rack. Once cooled, use the parchment paper overhang to lift the bars out of the pan. Cut into squares and serve.