Preheat the oven: Begin by preheating your oven to 350°F (175°C) and greasing a 9x13 inch baking pan to ensure easy removal of the bars later.
Cream the butter and sugars: In a large mixing bowl, use an electric mixer to cream together 1 cup unsalted butter, 1 cup granulated sugar, and 1 cup brown sugar until the mixture is smooth and fluffy, approximately 3-4 minutes.
Add the eggs and vanilla: Incorporate 2 large eggs and 1 tsp vanilla extract into the creamed mixture, mixing well until completely combined.
Combine the dry ingredients: In another bowl, whisk together 2 cups all-purpose flour, 1 tsp baking powder, 1/2 tsp baking soda, and 1/2 tsp salt until evenly mixed.
Mix dry into wet: Gradually add the dry ingredients to the creamed mixture, mixing on low speed until just combined. Avoid overmixing to keep the bars tender.
Fold in the blueberries: Gently fold in 1 cup fresh or frozen blueberries, the zest & juice of 1 lemon, ensuring even distribution without breaking the berries.
Spread the batter: Pour the batter into the prepared pan, using a spatula to smooth it evenly.
Bake: Bake in the preheated oven for 30-35 minutes. The bars are done when a toothpick inserted into the center comes out clean.
Cool and drizzle: Allow the bars to cool in the pan for about 10 minutes. Once slightly cooled, melt 1 cup white chocolate chips in the microwave or over a double boiler, then drizzle over the cooled bars.
Cut and serve: Allow the chocolate to set before cutting the bars into squares for serving. Enjoy!