Equally stagger two racks in the oven and preheat it to 375°F. Line two baking sheets with parchment paper.

In a stand mixer fitted with the paddle attachment, cream together the butter, brown sugar, and granulated sugar until smooth, 2 to 3 minutes.

Add the rum, coffee, and mascarpone and beat on medium speed until the dough noticeably lightens in color and doubles in size, another 3 to 4 minutes. It will look whipped.

While the wet ingredients are mixing, in a small bowl, whisk together the flour, ½ teaspoon kosher salt, and baking powder.

In two rounds, add the dry ingredients to the wet, with the mixer at its lowest speed to combine.

Add the chocolate to the dough and fold this in by hand with a spatula.

Roll the dough into 21 tablespoon-size balls (25 grams each). Space the cookies 2 inches apart on lined baking sheets. Indent each in the center to encourage equal spreading. Sprinkle with the lightest touch of flaky salt.

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

Remove the cookies from the oven and give the trays a few smacks on the counter to flatten the cookies out.

While the cookies are still warm, use a cookie cutter or biscuit cutter to swirl around the cookies to make them into perfect circles (optional, for those who like perfection).

Let the cookies cool for 20 minutes.

Use a sifter or small sieve to dust the cocoa powder over the top before serving.