Prep the pan: Heat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment, leaving overhang for easy lifting.
Lightly grease.
Mix the wet ingredients: In a large bowl, whisk melted butter with granulated and brown sugars until glossy, 30–45 seconds. Whisk in eggs and vanilla until smooth.
Combine dry ingredients: In a separate bowl, whisk flour, baking powder, and salt. If using lemon zest, add it now.
Make the batter: Add dry ingredients to wet and fold just until no dry flour remains.
Batter will be thick.
Add the mix-ins: Fold in white chocolate. Gently fold in raspberries last to minimize breaking. It’s okay if some streaks form.
Pan and smooth: Scrape batter into the pan.
Spread evenly with an offset spatula. If desired, sprinkle a pinch of flaky sea salt on top.
Bake: Bake 24–28 minutes until edges are set and lightly golden, and a toothpick inserted near the center comes out with a few moist crumbs (not wet batter).
Cool and cut: Cool in the pan on a rack for at least 30–45 minutes. Lift out using parchment and cut into 16 squares.
For clean slices, chill 20 minutes before cutting.
Serve: Enjoy slightly warm or at room temperature. Great with ice cream or a dusting of powdered sugar.