Step 1

Heat the butter in a saucepan until it is all melted, dark amber in color, and the milk solids are caramelized. The butter should smell nice and nutty. Remove from heat and place in your bowl.

Step 2

Stir the instant espresso into the butter and set aside to cool.

Step 3

Mix together the flour, cornstarch, baking soda, and salt together in a small bowl and set aside.

Step 4

Beat the butter and both sugars until well-combined.

Step 5

Add in the egg, yolk, and vanilla. Beat well.

Step 6

Stir in the flour mixture and mix until just combined,

Step 7

Fold in the chopped chocolate.

Step 8

Portion the cookie dough into balls that are 2 tablespoons big and set aside to chill in the refrigerator for at least an hour, preferably overnight up to three days.

Step 9

Preheat the oven to 350°F. Line two baking sheets with parchment paper.

Step 10

Place the cookie dough 2-inches apart on your baking sheets and bake for 8 - 10 minutes, or until golden brown.

Step 11

Allow the cookies to rest on the baking sheet for one minute before transferring to a wire rack to cool completely. Top with flaky sea salt while still warm