Heat the oven to 350°F (175°C). Line a light-colored 8×8-inch metal pan with parchment paper.
Whisk the melted butter and brown sugar in a large bowl. Whisk in the maple syrup, egg, yolk, and vanilla until smooth.
Fold in the flour, baking powder, kosher salt, and cinnamon just until no dry streaks remain. Fold in ¾ cup toasted pecans.
Spread the batter evenly in the pan and top it with the remaining ¼ cup pecans. Bake for 20–22 minutes, until the top is set and a butter knife inserted in the center comes out clean.
Cool for 30 minutes. Whisk the glaze ingredients until smooth, drizzle over the blondies, and add optional flaky sea salt. Lift from the pan and cut into 16 bars.