Preheat oven to 350 degrees. Spray a 9 X 13-inch baking pan with cooking spray. Set aside.
Pulse 1/2 cup walnuts in a food processor until almost fine (do not over process). Whisk the ground nuts, flour, baking powder, baking soda and salt in a medium bowl.
Beat the butter and brown sugar 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 until just combined. Fold in the chocolate and the remaining 1/2 cup walnuts. Spread the batter in the prepared pan and bake until blondies are light brown around the edges and spring back when pressed, about 35 minutes (I had to leave mine in longer).
Meanwhile, make the topping. Place the maple syrup and butter in a small saucepan over medium heat and cook until the mixture bubbles and thickens, swirling the pan, about 6 minutes (mine never really thickened a whole lot). Add the cream and continue to cook until the mixture is the consistency of caramel (again, mine never really thickened until I removed it from the heat and it cooled slightly).
Cut the blondies into bars and serve topped with a scoop of vanilla ice cream, chopped walnuts, and a healthy drizzle of the maple-butter sauce.
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