Prepare the caramel: Melt the sugar in a small saucepan over medium heat until it turns into a golden caramel. Immediately pour into a greased 10-cup Bundt pan, tilting to coat the bottom. Set aside.
Make the flan: In a blender, combine sweetened condensed milk, evaporated milk, eggs, and vanilla extract. Blend until smooth. Set aside.
Make the chocolate cake batter:
In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt.
In another bowl, cream the butter and sugar until light and fluffy.
Beat in the egg and vanilla.
Alternately add dry ingredients and buttermilk, mixing until just combined.
Assemble the cake:
Pour the chocolate cake batter into the Bundt pan over the caramel.
Slowly pour the flan mixture on top. (It will float above the cake batter—this is normal.)
Bake in a water bath:
Place the Bundt pan in a larger roasting pan.
Fill the roasting pan with about 1 inch of hot water.
Cover the Bundt pan with foil and bake at 350°F (175°C) for 60 minutes, or until a toothpick inserted into the cake layer comes out clean.
Cool and chill:
Remove from water bath and let cool to room temperature.
Refrigerate for at least 4 hours or overnight.
Unmold and serve:
Carefully invert onto a serving plate. The caramel should drizzle beautifully over the flan. Slice and enjoy!