Cut the baguette on the bias into 8 thin slices. In a small bowl, mix together the chopped mint and butter. Cut the chocolate pieces so that 2 pieces match the size of the bread slices. Heat the chocolate in the microwave 5 seconds to soften just enough cut the pieces.
Spread the butter onto the slices of bread. Lay out 2 pieces of chocolate on each tartine. Sprinkle a little salt on top. Serve.