Preheat oven to 375℉ and line a baking tray with parchment paper.
In a large bowl, whisk together butter, espresso powder, granulated sugar and brown sugar.
Add egg and vanilla and whisk until smooth.
Add flour, baking soda and salt. Fold with a rubber spatula until just combined.
Add chocolate and fold until just combined.
Add mascarpone cheese and fold until slightly swirled. Be very careful not to over-swirl, it's better to mix too little than too much.
Using a cookie scoop, scoop in 1/4 cup cookies, placing each cookie at least 1 inch apart from the other cookies.
Bake cookies for 11-13 minutes, or until the center no longer looks wet. Cool on a wire rack.