Brush butter on one side of the slices of bread. On a work surface, put 4 slices of bread, buttered-side down, and sprinkle with chocolate up to 1/4 inch from the crust. Cover with remaining slices of bread, buttered-side up.
In a large striped pan or non-stick skillet, cook sandwiches at medium-low heat, 2 at the same time, 6 to 8 minutes or until chocolate has melted and the bread is golden (press them with a spatula and flip at mid-cooking). Cook remaining sandwiches the same way.
Cut sandwiches in 2 and sprinkle with icing sugar. Serve warm.