Arrange a rack in middle of oven; preheat to 175°C.
Grease a 10- to 12-cup Bundt pan with 30 g (2 tbsp) butter, making sure to get all crevices. Pour caramel sauce in bottom of pan; set aside.
In the large bowl of a stand mixer fitted with the whisk attachment (or in a large bowl with a hand mixer), beat brown sugar and remaining 113.5 g (1/2 cup + 2 tbsp) butter on medium-high speed until light and fluffy, 3 to 5 minutes. Beat in egg until combined.
In a medium bowl, whisk flour, cocoa powder, baking powder, baking soda, and salt. Add half of dry ingredients to egg mixture, beating until just combined. Pour in coffee and beat until incorporated. Add remaining dry ingredients and buttermilk and beat until just combined. Pour batter into prepared pan on top of caramel.
In a large bowl, whisk eggs, condensed milk, and vanilla until combined. Add milk and whisk until incorporated (some bubbles are okay). Slowly pour flan mixture over the back of a large spoon into Bundt pan (this reduces any disturbance to the cake batter).
Tightly cover pan with foil and place in a large roasting pan. Carefully transfer pans to oven, then pour in boiling water (about 710 ml (3 cups)) until it reaches halfway up the sides of Bundt pan. If you don’t have enough water because roasting pan is large, add more warm water from tap.
Bake chocoflan until a tester inserted into the center of cake comes out mostly clean, 60 to 70 minutes. (A few crumbs is okay as flan will set more while cooling.) Carefully transfer Bundt pan to a wire rack and uncover. Let cool 1 hour, then refrigerate at least 1.5 hours before serving (if refrigerating longer than 1.5 hours, cover pan with foil).
Loosen cake by placing Bundt pan in a large bowl of hot water. Place a platter upside down on cake pan, then flip to unmold cake. Spoon any remaining caramel from pan over top of cake.
Chocoflan can be made 2.25 days ahead. Store in an airtight container and refrigerate.