Preheat the oven to 350˚F. Line two large baking sheets with parchment paper or silicone baking mats.

Combine flour, baking soda, and salt in a medium bowl. Whisk to combined and set aside.

In a separate large mixing bowl, add cooled melted butter, brown sugar, granulated sugar, and espresso powder. Stir well to combine completely. The mixture should look ike wet sand at this point. Add in egg, egg yolk, and vanilla extract, stirring to combine.

Add dry ingredients and stir until just combined. Do not over mix. Fold in chopped dark chocolate.

Using a cookie dough scoop, scoop out dough in even amounts. The balls should be about 2 Tbsp in size each, if you don't have a scoop. Place dough balls on one lined baking sheet and chill in the refrigerator for 10 minutes (or in the freezer for 5 minutes).

To get the gooey pools of chocolate on top, slightly flatten each ball of dough with the palm of your hand. Press a couple of chunks of chocolate into the top of each ball. Form them back into balls and place on baking sheets, spacing them apart about 2 inches.

Bake for 10 to 12 minutes, until the edges are set and the centers are a little puffed. Remove from the oven and sprinkle with flaky sea salt. Let cool on the pans for 5 minutes before removing to wire racks to cool completely.

Store at room temperature in an airtight container for up to 4 days.