Preheat the oven to 400°F.
In a stand mixer, add the softened butter, brown sugar and white sugar. Cream together for 2-3 minutes or until smooth.
Add in the rum, espresso coffee grounds and mascarpone. Beat on medium speed for another 2-4 minutes, you should see the dough noticeably lighten in color and double in size. It will look “whipped.”
While the stand mixer is mixing, whisk together the flour, cornstarch, salt and baking powder.
In two rounds add the dry ingredients to the wet, using the mixer on low to combine.
Chop your chocolate into small slivers and add to the dough. Fold this in with a spatula.
Scoop the dough into 8 equal size balls, all around 4 ounces each. Note that these are BIG cookies, so see recipe notes on how to make a smaller size.
Bake the cookies for 10 minutes at 400°F on a parchment lined baking sheet, or until the tops are crinkly and the cookies have spread. They will look underdone, but that’s the point!
Remove the cookies from the oven and give them a few smacks on the counter to flatten them out. Use a cup or bowl to swirl around the cookies to make them into perfect circles. Do this by putting the cup over the cookie as if you’re trying to trap it. Then swirl!
Keep the cookies on the parchment paper but move them to a cooling rack. They will be very tender, but the residual heat will continue to “cook” them. Let them cool for 20 minutes, and you’ll be left with a soft, almost creamy cookie texture that will remind you of tiramisu!
Note – you can also make 16 cookies, each about 2 ounces each. With cookies this small it will be hard to get the perfect “creamy” center, but to ensure the best results, bake for 7-8 minutes at 400°F and immediately move to a cooling rack!