2 tablespoons unsalted butter, plus more for the ramekins
1 tablespoon sugar, plus more for the ramekins
1/2 cup chocolate syrup
1 large egg yolk plus 3 egg whites
Pinch of cream of tartar
Cocoa powder, for dusting (optional)
Preheat the oven to 400 degrees F. Butter four 6-ounce ramekins; sprinkle with sugar and tap out the excess. Put the ramekins on a baking sheet; transfer to the freezer while you make the batter.
Heat the chocolate syrup and 2 tablespoons butter in a small saucepan over low heat until the butter melts, about 2 minutes; let cool. Whisk the egg yolk in a medium bowl. Slowly whisk in the chocolate syrup mixture until combined.
Beat the egg whites with a mixer on medium speed until foamy. Add the cream of tartar and beat until soft peaks form, about 2 minutes. Add 1 tablespoon sugar and beat until stiff peaks just form, 1 to 2 more minutes.
Fold one-third of the beaten egg whites into the chocolate-yolk mixture until combined. Fold in the remaining egg whites in two additions until no white streaks remain.
5. Divide the batter among the prepared ramekins. Bake until the souffles are puffed and browned around the edges, 18 to 20 minutes. Dust with cocoa powder and serve immediately.