• Preheat oven to 350 degrees F. Line an 8-inch square metal pan with parchment paper.
• Melt the butter and let it cool for about 5 minutes.
• Whisk the melted butter and brown sugar together until smooth and glossy.
• Add the egg, yolk, and vanilla and whisk until well blended. If using maple extract, whisk that in too.
• Mix the flour, cornstarch (if using), baking powder, baking soda, and salt together in a a separate bowl, then add to the batter and stir just until combined.
• Fold in the walnuts.
• Spread batter evenly in the pan and bake for 20 to 24 minutes, until the edges are set and the center is still slightly soft.
• Let cool in the pan for about 20 to 30 minutes before slicing.

Maple-Butter Sauce

• To make the maple-butter sauce, cut the butter into chunks and put it in a small saucepan. Heat over medium until it is about halfway melted, then add the maple syrup and stir until smooth. Return to the heat and cook over medium for about 4 minutes, swirling the pan often so it won't burn, until mixture thickens slightly and becomes fragrant.
• Remove from heat and add the cream, then return to heat and cook, allowing it to come to a very gentle boil, just until it thickens slightly. Again, be careful not to let it burn. Remove from heat. Let cool slightly and add a pinch or two of salt. If making ahead, you can chill it, but it will firm up and need to be warmed again.
• Serve blondies slightly warm with vanilla ice cream and spoon the maple sauce over the top.