Preheat the grill to 350F (180C). If using a charcoal grill, put the coals on one side of the grill. Butter and flour a 9-1/2 inch (24cm) round cake pan.

Place the chocolate and the butter in a small pan over low heat and melt it, stirring occasionally to be sure the chocolate isn’t sticking. When the mixture is melted, remove it from the heat to let cool slightly.

In a large bowl or the bowl of an electric mixer, whisk the eggs until they are combined. Add the sugar and continue whisking until they are pale yellow and slightly foamy, which will take several minutes.

Fold the chocolate mixture into the eggs and sugar until thoroughly combined. Pour the batter into the prepared pan, holding the bowl low over the pan so that the mixture doesn't have to "fall" too far.

Set the pan on the grill away from the coals. Cover the grill and bake until the cake is puffed and slightly firm to the touch on top, which will take 25 to 30 minutes. Remove the cake from the grill and let cool to lukewarm before slicing.