Preheat oven to 400°F. Butter and flour a 9-inch springform pan.
Place chocolate in a large bowl and microwave for 3 minutes.
Add butter and microwave 1 minute more.
Stir until smooth.
Stir in sugar.
Stir in flour.
Stir in eggs.
Pour batter into prepared pan. Place on a baking sheet to catch any leaking butter, and place in oven. Reduce heat to 350°F.
Bake 25 minutes. The cake will still be liquidy in the center.
Let cool completely before unmolding.