Prep the pan: Heat oven to 350°F (175°C). Line an 8-inch square pan with parchment, leaving overhang for easy lifting.
Lightly grease the parchment.
Mix wet ingredients: In a medium bowl, whisk melted butter and brown sugar until glossy and smooth, about 30 seconds. Whisk in the egg and vanilla until fully combined.
Combine dry ingredients: In a separate small bowl, stir together flour, baking powder, and salt. Add to the wet mixture and fold with a spatula until just combined.
Do not overmix.
Add mix-ins: Fold in white chocolate chips (and nuts, if using), reserving a small handful to sprinkle on top.
Spread and top: Scrape the thick batter into the prepared pan and smooth the surface. Sprinkle the reserved chips over the top for a pretty finish.
Bake: Bake for 20–25 minutes, until the edges are set and lightly golden and the center looks slightly underdone but not jiggly. A toothpick should come out with a few moist crumbs.
Cool completely: Let the blondies cool in the pan on a rack for at least 1 hour.
Lift out using the parchment and slice into squares.