For the Cake
1. Preheat oven to 350 degrees.
2. Melt the chocolate and butter together in a medium bowl set over a simmering pot of water. Remove from the heat and stir in the sugar, vanilla, eggs and cocoa. Mix well.
3. Place ice cream cones on a baking sheet and fill with batter, leaving a small space at the top to allow for the rising of the cake. Bake for 20 minutes. Cool completely before adding the marshmallow.
For the Marshmellow 'Ice Cream
1. Pour the water, sugar and syrup into a small pot. Sprinkle the gelatin over the surface of the mixture and gently whisk the mixture until the gelatin and sugar dissolves.
2. Bring the mixture to a simmer, heating it just long enough to melt the gelatin and sugar. It does not need to come to a simmer but it does need to get hot to the touch. Rub some of the mixture between your fingers. If it does not feel gritty and the mixture is very hot move on to the next step.
3. Add the marshmallows, vanilla and optional flavoring and using a stand mixer begin whipping the mixture on high. At this point you have two options. You may whip until the marshmallows look like soft whipped cream and the bowl feels cools to the touch then pour it into a lightly oiled 9x13 baking pan until it sets then cut it into squares. You may also continue whipping until the marshmallow resembles a stiff meringue and easily holds its shape in stiff peaks then pipe it out.
4. As the marshmallow cools the gelatin begins to firm up allowing the mixture to absorb air and increase in volume. To test the degree of whipping, dip a spoon into the mixture then remove it and invert it. Watch how the mixture slumps off the spoon. For best results it should be beaten past the point that it holds a soft peak almost to the point that it has stiffened and holds a stiff peak…
5. Working quickly spoon the marshmallow into a zippered plastic bag and cut off one corner. Pipe onto top of ice cream cones, and garnish with candy sprinkles.