Combine the chocolate and butter in a microwave-safe bowl. Heat at 50 percent power for 2 minutes; stir, then microwave at 30-second intervals until completely melted. Whisk in the cocoa and 1 3/4 cups sugar until smooth. Microwave the mixture on full power for 5 minutes, stopping to whisk occasionally. (Don't worry if the mixture looks grainy; continue to cook and whisk until smooth.) Add 1 cup half-and-half and microwave on full power until smooth and slightly fudgy, 2 minutes. Transfer to a thermos to keep warm.
Fold the remaining 2 tablespoons half-and-half and 2 tablespoons sugar into the mascarpone in a bowl; cover and refrigerate.
Preheat a grill to medium. Brush the cake slices lightly with oil and grill for 1 to 2 minutes per side. Brush the pineapple lightly with oil and grill until marked, about 1 minute per side.
Top the cake with pineapple, a scoop of mascarpone cream and hot fudge. Garnish with coconut and/or nuts, if desired.
Photograph by Rick Lew
Recipe courtesy Duff Goldman for Food Network Magazine