Preheat oven to 375 degrees.

In a stand mixer with the paddle attachment, mix together the butter and sugars until creamed. Then add rum, coffee, and mascarpone cheese. Beat on medium another 3-4 minutes, until the mixture appears whipped.

In a separate bowl combine flour, salt, and baking powder. Add 1/2 of the dry ingredients to the wet ingredients and mix on low. Once mixed, add the other 1/2 of the dry ingredients and continue to mix on low until well mixed.

Fold the dark chocolate into the dough by hand with a wooden spoon or spatula.

Roll into tablespoon size balls and space out 2 inches apart on a prepared baking sheet. Gently press each ball in the center to encourage equal spreading, and lightly sprinkle flaky salt on each cookie.

Bake for 9-10 minutes, until the tops are crinkly and the cookies have spread.