Prepare the pan: Preheat the oven to 375°F (190°C). Line an 8x8-inch (20x20 cm) baking tin with parchment paper.
Mix wet ingredients: In a large bowl, cream together the butter and brown sugar until smooth. Beat in the egg, vanilla, and pumpkin purée until well combined.
Add dry ingredients: Stir in the flour, baking powder, pumpkin spice, cinnamon, and salt. Mix until just combined.
Fold in chocolate chips: Stir in the white chocolate chips, then spread the batter evenly into the prepared baking pan.
Bake: Bake for 20–25 minutes, or until a toothpick inserted in the center comes out mostly clean. Remove from the oven and cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
Prepare topping: Melt the chopped white chocolate in the microwave in 30-second bursts, stirring between each round, until smooth. Set aside 2 tablespoons of the melted chocolate in a small bowl.
Dip and drizzle: Dip the tops of the cooled blondie fingers into the melted white chocolate and place them back on the wire rack. Stir the reserved 2 tablespoons of white chocolate with 1 tablespoon pumpkin purée, then drizzle or pipe over the dipped blondies for a festive finish.
Set and serve: Let the chocolate set completely before slicing into 12 finger-sized bars. Enjoy!