Preheat oven to 350°F. Line a loaf pan with parchment, leaving overhang on the sides.
Whisk together melted butter and brown sugar until glossy and smooth. Add egg yolk and vanilla and whisk until thickened slightly.
Stir in flour, baking powder, and salt just until combined. Do not overmix.
Fold in pistachios and white chocolate chips.
Spread evenly into the pan and smooth the top.
Bake 20 to 22 minutes, until the top is lightly golden and the center looks just set. A toothpick should come out with a few moist crumbs, not wet batter.