In a medium bowl, mix together the mascarpone, horseradish, ketchup and 1/2 teaspoon salt with a rubber spatula. Cover and refrigerate until ready to use.
Allow the flank steak to come to room temperature for 30 minutes before grilling. Preheat a grill or grill pan to medium high.
Meanwhile, mix together the cocoa powder, coffee, cumin and paprika in a small bowl. Sprinkle the steak on both sides with 1 1/2 teaspoons salt and rub with the cocoa mixture, followed by the olive oil (to help the spices stick).
Grill the steak for about 8 minutes per side, until an instant-read thermometer registers 125 degrees F in the thickest part. Remove from the grill and let rest for at least 10 minutes.
Meanwhile, cut the ciabatta rolls through the equator and grill, cut-side down, until golden brown and nicely marked, about 1 minute. Spread the rolls with the mascarpone mixture.
Thinly slice the steak against the grain and pile onto the rolls. Slice the sandwiches in half and serve.