Preheat oven to 350°. Line a 9x13 baking pan with foil, leaving a 2-inch overhang on the two short sides. Lightly grease the foil with butter.

Stir the flour, salt and baking powder together, set aside. Place the butter in a medium saucepan, pour white chocolate chips over the butter (having the butter on the bottom will prevent the chocolate from burning). Heat over medium-low heat, stirring occasionally until melted and smooth. Remove from heat, stir in the sugar, eggs, and vanilla. Stir in the flour mixture until smooth. Spread the batter into the pan, using fingers to gently press and spread the batter evenly. Bake until the bars 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, 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 the pan away from any heat and let the bars cool in the pan for a few minutes while they set. Remove the bars using the overhang, set on a cutting board to cool.

Once cool, cut into small squares. Using a large knife dipped in hot water will allow for a nice clean cut. Dust with powdered sugar prior to serving.