Line a 9 x13 inch baking pan with foil, leaving an overhang on two sides; butter the foil.
Pulse 1/2 cup of the walnuts in a food processor until almost fine (do not overprocess). Whisk the ground walnuts together with the flour, baking powder, baking soda and salt in a bowl.
In another bowl, beat butter and brown sugar together with a mixer on medium speed until fluffy, about 2 minutes. Beat in the eggs, one at a time, scraping down the bowl with a rubber spatula. Beat in the vanilla.
Gradually beat in the dry ingredients just until combined. Fold in the white chocolate and the remaining 1/2 cup walnut pieces.
Spread the batter in the prepared pan and bake for 30 minutes until light brown around the edges and it spring back when pressed. Cool slightly on a rack.
In the meantime, make the topping by adding the maple syrup and butter in a saucepan. Cook over medium heat until the mixture bubbles and thickens, swirling the pan, about 6 minutes. Add the cream and continue to cook until the mixture is the consistency of caramel.
Using the foil, lift the blondies out of the pan and cut into large bars. Serve each square warm topped with ice cream, the maple-butter sauce and some of the 1/2 cup walnuts for topping. Makes 9 servings.
