Preheat an oven to 375 degrees and grease 4 ramekins with butter or nonstick spray.
Prepare a double boiler over medium-low heat. Combine the chocolate with the butter in a medium bowl and melt, stirring occasionally, until melted and smooth. Remove from heat.
While the chocolate is melting, whisk eggs and sugar together until pale yellow and thickened.
Fold chocolate, egg mixture and salt together until just combine. Divide batter among buttered ramekins and bake until the edges are set but the center is underdone, about 10 minutes.