Preheat oven to 325. Butter an 8" cake pan, line with parchment paper, and butter the parchment. Place cake pan into a roasting pan.
Combine sugar and water in a small saucepan and bring to a rolling boil until sugar is dissolved.
Take off heat and add butter and chocolate. Let sit 2 minutes to melt, stir until smooth.
Whisk in liqueur, then eggs, one at a time.
Pour batter into prepared cake pan. Add 1 1/2 c warm tap water to roasting pan and bake on middle rack of preheated oven until slightly puffed and soft in the middle - approximately 40 minutes. Remove from waterbath and cool on wire rack 1 hr. Unmold onto serving plate.
If using same day, don't refrigerate. If making ahead, wrap with plastic and chill in fridge. Bring to room temperature before serving.