ARRANGE an oven rack in the center position and PREHEAT to 350°F. LINE the inside of an 8×8-inch pan with aluminum foil and BUTTER the foil; SET aside.
PLACE 5 oz. of white chocolate in the bowl of a food processor.
COOK 10 Tbsp. of butter in a small saucepan over medium heat, STIRRING until the butter SPUTTERS and FOAMS and the solids turn golden brown, about 5 to 7 minutes. SCRAPE the butter and all the browned bits into the food processor on top of the white chocolate. LET it sit for about 5 minutes to allow the chocolate to melt. (RESERVE the saucepan to make the glaze.)
ADD the chestnuts, granulated sugar, brown sugar, whole eggs, egg yolk and vanilla to the food processor. PROCESS in long pulses until the mixture is very smooth and the chestnuts are broken down. (The chestnut bits add texture).
ADD the flour and 1 tsp. salt to the food processor and PULSE a few more times just until mixed in. REMOVE the lid and FOLD the batter a couple of times with a spatula to make sure it’s evenly mixed. SCRAPE into the prepared pan and SMOOTH the top in an even layer, spreading into the corners.
BAKE until the edges are puffed and the center is firm to the touch but still shiny, about 20 to 25 minutes. REMOVE from the oven and COOL in the pan 20 minutes, then USE the foil to lift the blondies out of the pan. TRANSFER to a wire rack to cool completely.
For Glaze: COOK 2 Tbsp. butter in the reserved saucepan over medium-low heat. WHISK until the butter turns golden, about 3 minutes. REMOVE from the heat.
ADD 2 oz. white chocolate to the saucepan and WHISK until melted and the mixture is smooth. SET the saucepan aside to cool, whisking occasionally, until the mixture is lukewarm and thickened but still liquid.
WHISK in the powdered sugar and pinch of salt until smooth, then POUR over the surface of the blondies. USE a spoon to spread the glaze all the way to the edges in a smooth, even layer.
SPRINKLE with decorative sugar or sprinkles and CHILL until glaze is completely set, about ½ hour CUT into a 6×6 grid to make small squares, and SERVE.
Watch the butter continually while browning because it can sputter out of control and will eventually burn.
Double the recipe if you’re baking for a crowd and bake the blondies in a 9×13-inch pan. You will need to increase the bake time a bit to compensate for the larger quantity of batter.