Preheat the oven to 350 degrees F. Line a 9X13-inch baking pan (I use metal) with foil (for easier cleanup). Grease the foil with nonstick cooking spray. Set aside.
For the white chocolate brownies, combine the butter and white chocolate in a microwave-safe bowl and heat for 1-minute increments, stirring in between, until smooth.
Stir in the granulated sugar, eggs, vanilla extract, and lemon zest and mix until evenly and well-combined. Stir in the flour, salt and baking powder until smooth.
Spread the batter evenly in the bottom of the prepared pan. Bake for 20-22 minutes until the brownies are just slightly set but still soft in the center.
For the lemon bar layer, in a medium bowl, whisk together the eggs, sugar, lemon zest, lemon juice, and flour until smooth and well-combined.
Pour the lemon mixture over the top of the warm white chocolate brownies. Return to the oven and bake until the lemon layer is set, about 18-20 minutes. It's ok if there are bubbles on the surface of the lemon layer. They won't look very pretty coming out of the oven but they'll get pretty in just a sec.
Let the bars cool completely in the pan. Refrigerate until ready to serve. When ready to serve, dust the top of the bars with powdered sugar. Cut into squares and serve immediately (the powdered sugar melts into the bars the longer they sit, so dust only when ready to serve).
White Chocolate: white chocolate chips are notoriously bad melters. So I recommend using the white bar chocolate (NOT candy coating or almond bark) for this recipe. I can easily find the Ghirardelli brand at my local grocery store (right above the chocolate chips).Lemon Juice: fresh lemon juice makes all the difference and will produce a much brighter flavor than bottled lemon juice.