In a medium saucepan, heat the pan over high heat, combine the water and butter, and bring to a boil.
Add the nescafe powder and stir until dissolved.
Add the chocolate, sugar, and vanilla to a medium bowl and pour the hot butter mixture over it.
Whisk until the chocolate is melted and the mixture is glossy.
Add the eggs and whisk well to combine.
Pour the filling into the pre-baked shell. Bake for 12-15 minutes, or until chocolate is mostly set but the center jiggles slightly.
Cool the tart for at least 30 minutes, or until cool to touch.