Preheat oven to 425 degrees F. Butter and flour 6 (4 oz) ceramic ramekins; transfer to a rimmed baking sheet. Using an electric mixer, beat egg yolks and egg in a medium bowl until doubled in volume and eggs hold a slight ribbon, about 2 1/2 minutes.

Add dulce de leche; beat until well blended, then gradually add 2 1/2 tablespoons flour; beat until well blended. Divide batter among ramekins.

Transfer sheet to oven. Bake cakes until outsides are golden brown in color but the centers still jiggle, 12–14 minutes.

Transfer sheet to a rack. Run a knife around edges of ramekins to loosen cakes; invert onto plates or into shallow bowls. Serve hot with ice cream.