Lightly grease the parchment.

Brown the butter: Melt the butter in a light-colored saucepan over medium heat. Cook, swirling, until it foams and smells nutty and golden with brown flecks, 4–6 minutes. Pour into a mixing bowl and cool 5–7 minutes so it doesn’t scramble the eggs.

Mix sugars and butter: Whisk the brown sugar and granulated sugar into the warm butter until glossy and combined.

Add eggs and flavor: Whisk in the egg, egg yolk, and vanilla until the mixture looks thick and smooth.

Stir in the lemon zest and lemon juice.

Combine dry ingredients: In a small bowl, whisk the flour, baking powder, and salt. Add to the wet ingredients and fold gently until just combined. Do not overmix.

Fold in mix-ins: Stir in the white chocolate and most of the pastel candy, reserving a small handful for the top.

Fill the pan: Scrape the batter into the prepared pan and smooth the top.

Sprinkle on the remaining candy for a festive finish.

Bake: Bake 20–25 minutes, until the edges are set and lightly golden and the center looks just barely done. A toothpick should come out with a few moist crumbs, not wet batter.

Cool and lift: Cool in the pan on a rack for at least 30–45 minutes. Use the parchment to lift out and slice into squares.

For the cleanest cuts, chill briefly before slicing.

Finish (optional): Dust with powdered sugar or add a pinch of fresh lemon zest over the top for extra brightness.