Preheat your oven to 350°F (175°C) and grease a 9×13-inch baking pan or line with parchment paper.

In a large bowl, mix melted butter and brown sugar until smooth. Add eggs one at a time, then stir in vanilla extract.

In a separate bowl, whisk flour, baking powder, and salt. Gradually fold into wet ingredients until just combined.

Toss blueberries in 1 tablespoon of flour and gently fold into the batter along with white chocolate chips if using.

Pour batter into the prepared pan and bake for 25-30 minutes, checking at 25 minutes for a slightly soft center.

Cool for at least 15 minutes, then slice into squares and serve warm or at room temperature