It is recommended that the oven be preheated to 400 degrees Fahrenheit.
Cream together the softened butter, brown sugar, and white sugar in a stand mixer for 2-3 minutes.
The dough will lighten in color and double in size once you add the rum, espresso coffee grounds, and mascarpone. Continue beating on medium speed for an additional 2-4 minutes.
The flour, cornstarch, salt, and baking powder should be whisked together while the stand mixer is mixing.
Using a mixer on low, combine the dry and wet ingredients in two rounds.
Add the chocolate slivers to the dough and fold them in with a spatula.
The cookies should be baked at 400°F for 10 minutes, or until the tops are crinkly and the cookies have spread. They will look underdone, but that's okay!
Use a cup or bowl to swirl around the cookies to make them into perfect circles. Put the cup over the cookie as if you're trying to trap it. Then swirl!
Let the cookies cool for 20 minutes on a cooling rack. They will be very tender at this point, but the residual heat will continue to "cook" them.
Enjoy your Tiramisu Cookies.