Preheat the oven to 350℉. Line two baking sheets with parchment paper.
In the bowl of a stand mixer, fitted with the paddle attachment, combine the melted butter and espresso powder. Mix until the espresso has melted and the mixture is smooth. Leave it to cool to room temperature.
1 cup unsalted butter, melted and very warm, 1 tablespoon instant espresso powder
In a medium bowl, whisk together the flour, baking soda, baking powder and salt.
2 ⅔ cups all purpose flour, 1 teaspoon baking soda, ½ teaspoon baking powder, ½ teaspoon fine sea salt
To the cooled butter and espresso mixture, on medium low speed, beat in the brown sugar and granulated sugar for about 5 minutes or until the mixture is well blended and cohesive.
1 cup light brown sugar, ¼ cup granulated sugar
Beat in the egg, egg yolk and vanilla until thoroughly combined.
1 large egg, 1 large egg yolk, 2 teaspoons pure vanilla extract
On low speed, beat in the flour mixture just until combined. Scrape down the sides and bottom of the bowl, then beat again for 30 more seconds.
With a large cookie scoop (I use #20, filling it about ¾ full) scoop the cookie dough, placing 6 dough balls per baking sheet. You should have 12 cookies total.
Bake the cookies for 13-15 minutes, until the cookies are golden around the edges and the centers no longer look wet. Cool completely.