Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan and line the bottom with parchment paper. Drizzle the caramel sauce over the parchment paper and set aside.

In a large mixing bowl, cream together the softened butter and brown sugar until light and fluffy. Add the egg and mix well.

In a separate bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt. Gradually add the dry ingredients to the butter mixture, alternating with the brewed coffee and buttermilk, mixing just until combined.

Pour the chocolate cake batter into the prepared pan over the caramel sauce.

In a separate bowl, whisk the eggs, sweetened condensed milk, vanilla extract, and whole milk (or half-and-half) until smooth. Pour the flan mixture carefully over the chocolate cake batter.

Cover the pan with foil and bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the cake comes out clean and the flan is set.

Let the chocoflan cool completely at room temperature, then refrigerate for at least 4 hours, or overnight for best results.

To serve, run a knife around the edges of the pan, then carefully invert the cake onto a serving plate. The caramel will drizzle down over the flan and cake, creating a beautiful presentation.