To brown the butter, add unsalted butter to a skillet or saucepan and set it over medium-low heat. Stir occasionally as it melts, then begin whisking continuously once it starts to sputter and foam.

After about 4-5 minutes, you’ll see golden brown specks forming on the bottom of the pan and the butter will turn a rich amber color. As soon as this happens, remove it from the heat and pour it into a heat-safe bowl to stop the cooking. Let it cool for at least 10-15 minutes before using.

While the butter cools, preheat your oven to 350°F (180°C). Line a 9x9-inch baking pan with parchment paper, leaving an overhang on two sides.

In a large mixing bowl, whisk together the cooled brown butter, granulated sugar, light brown sugar, and pure maple syrup until well combined.

Add the large egg, egg yolk, mustard, and vanilla extract. Whisk until the mixture is smooth and glossy.

Add flour, baking powder, baking soda, ground cinnamon, nutmeg, allspice, and salt to the wet mixture. Stir gently with a rubber spatula or wooden spoon until just combined. The batter will be thick and glossy.

Remove the pan from the oven and place it on a wire rack. Let the blondies cool in the pan for about 10 minutes, then use the parchment overhang to lift them out and let them cool completely on the rack before glazing, about 1 hour total.

In a small bowl, whisk together powdered sugar, pure maple syrup, and a splash of heavy cream until smooth and pourable. Adjust the consistency by adding more cream for a thinner glaze or more powdered sugar for a thicker one.

Drizzle the icing over the cooled blondies. Once the glaze is set, slice into squares and enjoy!