Flan Layer: Place a pot on the stove, pour in the milk and sweetened condensed milk, and heat on low until warm. In a bowl, crack 2 eggs and gently stir without beating them too much; the fewer bubbles, the better. Pour the warm milk mixture into the eggs, stir well, and strain through a sieve to remove any lumps.

Preheat the oven to 300°F (150°C) with heat from both top and bottom.

Caramel Layer: Pour the sugar into a pot and add enough water to cover the sugar. Heat on medium without stirring. Once the sugar turns a golden amber color, immediately add the lemon juice. This step helps prevent the sugar from darkening too quickly and reduces bitterness. If you prefer a darker color and a slightly bitter taste, let it cook a bit longer before adding the lemon juice, but avoid burning it.

Turn off the heat and quickly pour the caramel into a mold (it can be a ramekin, a 20cm round mold, or a loaf pan, use whatever mold you have). Let it cool until the caramel hardens. It's not necessary for the caramel to cover the mold evenly because it will spread during baking.

Gateau Layer: In a bowl, mix the flour and cocoa powder. In another bowl, mix the oil and milk. Crack 2 eggs into a bowl, divide the sugar into two parts, and add them to the eggs one at a time. Use a mixer on low speed, then gradually increase to high speed until the mixture is fluffy. Slowly add the milk and oil mixture while mixing on low speed until combined. The mixture should fall like ribbons when lifted. Sift the flour in 2-3 batches, folding gently to avoid deflating the batter, until all the flour is incorporated.

Pour the flan mixture into the mold with the caramel, then pour the cake batter on top of the flan. The batter is light and won't sink into the flan. Place the mold on a towel-lined baking tray, pour boiling water into the tray, and bake at 300°F (150°C) for about 55-60 minutes, depending on the mold. To check if the cake is done, insert a toothpick; it should come out clean.