Melt the chocolate with the butter in the microwave in three 30-second intervals.
Meanwhile, in a mixing bowl, quickly beat the eggs with the sugar, then mix in the flour.
Stir in the melted chocolate and butter just enough to achieve a smooth batter.
Pour the batter into a buttered and floured cake mold, or line the mold with parchment paper if you prefer.
Bake for 25 minutes at 160°C (320°F). Wait 5 minutes before unmolding and enjoy indulgently while warm to appreciate the flowing chocolate heart.