Heat oven to 350 degrees with a rack in the center. Rub a pat of butter all over an 8-inch square baking pan.
Whisk the flour, baking powder and salt in a small bowl.
With a flexible spatula, stir the brown sugar and butter in a large bowl until smooth. Add the egg and vanilla, and stir vigorously until smooth again. Add the flour mixture and stir gently just until no floury streaks remain, then stir in ¼ cup/40 grams sprinkles.
Spread the batter in the buttered pan into an even layer. Scatter sprinkles all over the top.
Bake until golden brown and starting to pull away from the edges of the pan, about 25 minutes. Cool completely in the pan on a rack. Cut into squares or rectangles. The blondies will keep in an airtight container for up to 3 days at room temperature and 3 months in the freezer.