Preheat the oven to 350 degrees and line an 8x8 inch baking pan with parchment paper.
Melt the butter in a medium bowl, then stir in the brown sugar until smooth.
Add the egg and vanilla extract to the butter mixture and stir until fully combined.
In a separate bowl, whisk together the flour, baking powder, and salt.
Fold the dry ingredients into the wet mixture until no streaks remain.
Stir in the white chocolate chips.
Spread the batter evenly in the prepared pan.
Drop spoonfuls of raspberry jam across the top and use a knife to swirl it gently through the batter.
Bake for 25 to 28 minutes until the edges are golden and the center is set.
Cool completely in the pan before slicing into squares.