Preheat the oven to 350°F (175°C). Prepare an 8×8-inch baking pan by spraying it with baking spray or lining it with parchment paper.
In a mixing bowl, whisk together the melted and cooled butter, packed dark brown sugar, and granulated sugar until smooth.
Add the large egg and vanilla extract to the butter‑sugar mixture and mix until combined.
Stir in the kosher salt, all-purpose flour, and baking powder until just combined; do not overmix.
Fold in the sprinkles gently, then spread the batter evenly into the prepared baking pan.
Bake for 25–30 minutes, until the center is set and the top is lightly golden.
Cool completely on a rack before cutting into 12 bars.