In a small bowl, combine the butter and cocoa to make "cocoa butter".
Line a large sheet pan with parchment paper. Place 1 sheet of phyllo on the pan and brush with cocoa butter. Sprinkle evenly with 2 tablespoons of sugar, then place another sheet of phyllo on top. Brush with cocoa butter, sprinkle with sugar, lay another sheet of phyllo on top, brush with cocoa butter, and sprinkle with sugar. Cover the stack with parchment paper. Place another pan on top to weight it down. Then place another, empty sheet pan on the very top. (This is to keep the phyllo from buckling during baking).
Bake for 15 to 20 minutes, until the phyllo is dark brown. It's very difficult to tell, as it's so chocolate-y looking as it is. Remove the whole stack of sheet pans and let cool (do not unstack the pans).
To assemble the dessert, break the phyllo up into random pieces about 3-inches square. Place a dab of mascarpone on each of 4 plates and press 1 phyllo piece down on it. Spoon on a heaping tablespoon of mascarpone in the center and surround it with raspberries. Place another phyllo piece on top and repeat, topping off with a final phyllo piece. Garnish with a mint sprig, a dusting of powdered sugar, and raspberry puree, if desired.