Instructions:

1. Prepare Oven and Water Bath Station:

 Preheat oven to 180°C (356°F, conventional setting) with the rack in the middle.

 Prepare a baking pan (large enough for six 6-ounce ramekins) by lining the bottom with a kitchen towel. Place six 6-ounce ramekins in the pan.

2. Make the Caramel:

 In a medium saucepan, add water, then slowly add sugar to the center. Do not stir.

 Bring to a boil over medium-high heat, swirling occasionally, for about 5 minutes.

 Cover with a lid for 2 minutes to wash down sugar crystals.

 Remove lid and continue heating until sugar caramelizes to a medium amber color, lowering heat towards the end to control caramelization.

 Immediately pour caramel into the ramekins, coating just the bottom. Be very careful as caramel is hot.

 Set aside to cool (cracking sounds are normal). Boil water in a kettle and let it cool to warm for the water bath.

3. Make the Flan Layer:

 In a medium bowl, whisk eggs, whole milk, sweetened condensed milk, and vanilla (if using) until combined.

 Pour mixture through a fine-mesh sieve into a measuring cup.

 Once caramel is cool, grease ramekin sides above the caramel. Pour flan mixture evenly into ramekins (approx. 79g/2.8 oz. each).

 Place ramekins in the prepared baking pan. Fill the pan with warm water (approx. 38°C/100°F) until halfway up the sides of the ramekins.

 Bake for 30-35 minutes, or until custard is set but still jiggly. Start cake batter 5-10 minutes before flan is done.

4. Make the Chocolate Cake Layer:

 In a medium bowl, mix flour, cocoa powder, baking powder, baking soda, and salt with a spoon.

 In a large bowl, whisk buttermilk, oil, egg, and orange zest (if using) until combined.

 Add sugar to wet ingredients and whisk to combine.

 Sift flour mixture over wet ingredients in two additions, whisking gently until just combined (don’t overmix; batter will look thick then become smooth).

5. Assemble and Bake Chocoflan:

 Remove pan with ramekins from oven. Test flan: if a drop of cake batter sinks, bake flan a few more minutes; if it sits on top, proceed.

 Carefully spoon chocolate batter over flan in each ramekin (approx. 47g each), spreading evenly.

 Loosely cover ramekins with parchment paper.

 Bake for 25-30 minutes, or until cake bounces back when lightly pressed and a toothpick inserted into the cake center comes out clean.

6. Cool and Unmold:

 Carefully remove ramekins from water bath and place on a wire rack.

 Cool completely at room temperature (about 1 hour).

 Cover well and chill in refrigerator for at least 4 hours, ideally overnight.

 To unmold: briefly dip bottom of a ramekin in boiling water. Gently shake ramekin until you feel liquid movement. Place a plate on top and invert to release. Drizzle any remaining caramel from ramekin over the chocoflan.