Make the brownie layer: Preheat the oven to 350˚. Line a 9-by-13-inch baking pan with foil, leaving a 2-inch overhang on the two short sides; lightly butter the foil. Melt the butter and white chocolate in a medium saucepan over medium-low heat, stirring often, until smooth. Remove from the heat and stir in the granulated sugar, eggs and vanilla. Stir in the flour, salt and baking powder until smooth. Spread the batter evenly in the prepared pan.
Bake until the brownies are lightly browned on top and softly set in the middle, about 25 minutes. Let cool 5 minutes.
Meanwhile, make the lemon layer: Whisk the eggs, granulated sugar, lemon juice and flour in a large bowl until smooth. Pour over the hot brownies. Return to the oven and bake until the lemon layer is set, about 20 minutes. Transfer to a rack and let cool completely in the pan.
Lift the brownies out of the pan using the foil overhang. Dust with confectioners' sugar. Cut into pieces, then decorate with yellow candies.