Start by melting butter in a small, heavy saucepan over medium heat. Make sure it doesn’t burn by using a thick-bottomed saucepan. Keep the heat at medium to ensure even browning, stirring constantly with a whisk or spatula for about 6 to 8 minutes to prevent milk solids from sticking.

As the butter melts, bubbles will appear, signaling the start of the browning process. Keep stirring until the butter turns golden brown. If the bubbling gets too intense, briefly take it off the heat to check the color accurately. You’ll notice a scent resembling toasted graham crackers or nuts.

Once you see golden-brown specks in the butter, take it off the heat immediately and transfer it to a heatproof bowl to stop the cooking. Remember, the butter will continue to brown even off the heat, so it’s better to be cautious.

Preheat the oven to 350º F. Line a 9×13 inch pan with aluminum foil or parchment paper.

In a mixing bowl combine the flour, cornstarch, baking soda, and salt. Set aside.

In a larger mixing bowl combine the browned butter, brown sugar, and granulated sugar. Beat on medium speed for 60 seconds.

Add in the eggs, egg yolk and vanilla extract, beat for 30 seconds.

Gradually mix in the dry ingredients, mixing on low speed. The dough will be thick.

Pour the batter into the prepared pan and bake at 350ºF for 25-30 minutes or until the top of the bars turn golden in color and the edges look set and puffy. Remove from the oven and allow the blondies to cool for 40 minutes before cutting into squares.