Preheat oven to 160 degrees. Grease a 6 cup bundt pan really well on all sides. Keep a large deep pan ready and settle the bundt pan in it.
Prepare the caramel first. Heat the sugar and water, swirling it till it boils and becomes an amber shade. Squeeze in the lemon juice and switch off.
Pour into the prepared pan and allow it to cool slightly.
Then prepare the cake. Whisk together ingredients from sugar to salt in a bowl.
Beat the wet ingredients till well combined. Pour into the dry ingredients and fold in till moist.
Spread the batter on top of the caramel sauce.
Whisk all the ingredients under "for the flan" vigorously till mixed well. Use only a whisk. Pour over the cake batter carefully.
Place the bundt pan into the large pan.
Pour hot water into the pan upto half of the bundt. Carefully slide into the oven.
Bake for 60 minutes till skewer comes out clean. Take out the pan and cool on a wire rack.
Cool in the fridge for two hours. To release, run on some hot water, tap the sides and push down onto a plate. Slice and serve.
Notes
You can use dulche de leche (DDL) as base instead of making the caramel. You can even omit this step. To make the whole dessert as chocolate, you may add two tablespoons of cocoa powder into the flan batter.