Melt the chocolate and butter in a heatproof bowl set over gently simmering water.

Whisk in the eggs gradually, stirring quickly to prevent curdling.

Fold in the flour and sugar substitute until the batter is smooth and uniform.

Divide the batter among porcelain ramekins, filling each three-quarters full.

Bake 7 to 10 minutes, depending on ramekin size, until the edges are set and the centers remain soft.

Serve immediately, warm from the oven.