Preheat the oven to 350 degrees F and line an 8-inch square baking pan with parchment paper.
Whisk the 1/2 cup melted vegan butter with the light brown sugar. Add the egg, egg yolk, and vanilla, then whisk until glossy.
Fold the flour, baking powder, and salt into the wet mixture just until combined. Spread the blondie batter evenly in the prepared pan.
Stir the 2 tablespoons melted vegan butter, dark brown sugar, cinnamon, and 1 tablespoon flour into a thick paste. Drop small spoonfuls over the batter and swirl them gently across the surface.
Bake for 24 to 28 minutes, until the edges are set and a tester inserted near the center comes out with moist crumbs. Cool completely in the pan.
Whisk the powdered sugar, oat milk, and vanilla until smooth. Add a few extra drops of oat milk only if needed for a thick, pourable glaze.
Drizzle the glaze over the cooled blondies and let it set for 20 minutes before portioning.
