Preheat an oven to 375 degrees Fahrenheit. Grease four ramekins and place on a baking sheet.
Melt the butter and chocolate together over a double boiler; cool over an ice bath.
Mix the sugar and eggs in a bowl until combined. Slowly pour the egg mixture into the chocolate, making sure the eggs don’t scramble.
Fold the flour in lightly until it just combines.
Pour into the ramekins and pop in the oven for 5-8 minutes. Watch them carefully. The outside should have started to pull away from the edge slightly, but the middle should still be shiny and viscous.
Let cool slightly, then run a knife along the edge of the ramekin to loosen cake. Place small plate over ramekin and flip it over quickly so the cake ends up on the plate.
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