Preheat oven to 375°F. Line a 9- by 9-inch baking pan with parchment paper or foil, allowing edges to hang over the sides like a sling. Grease parchment paper.

Stir butter with brown sugar and salt in large bowl. Stir in 1/2 cup maple syrup, egg and both extracts until combined.

Sift flour and baking soda and add to bowl. Stir with a rubber or wooden spatula until just combined. Stir in 1 1/2 cups walnuts.

Spread into prepared pan and smooth the top. Spread maple sugar or maple flakes over top. Sprinkle with remaining 1/4 cup walnuts.

Bake until golden and just set in center (but is still jiggly), about 24 minutes. Avoid over baking. Transfer pan to wire rack to cool, about an hour. Grab sling and lift blondies out of pan and cool completely.

Wrap in plastic and refrigerate for a couple of hours. Cut and serve at room temperature.