Melt sugar in a light-colored saucepan over medium heat without stirring; swirl occasionally until it turns amber, then pour into a greased Bundt pan.

In a large bowl, whisk together the dry chocolate cake ingredients.

In another bowl, beat the wet ingredients until smooth and combine with the dry ingredients, gradually adding hot water until the batter is thin and glossy.

Blend the flan ingredients in a blender until smooth, then carefully pour over the chocolate batter using a spoon to avoid mixing.

Set the cake pan in a larger roasting pan; add hot water to the roasting pan until it reaches halfway up the sides of the cake pan. Bake at 350°F (175°C) for 1 hour.

Let the cake cool completely, refrigerate for at least 4 hours, then unmold onto a serving plate.