Pre-heat the oven to 180C non-fan assisted (355F or 165C fan assisted). With soft butter, lightly grease a 23cm/9" cake tin, and line the base with a circle of parchment. Coat the sides with a tablespoon of cocoa powder and then tap out any excess. Set this to one side.

Into a medium saucepan, add the cocoa powder, espresso and water. Whisk it over a medium heat until it is steaming and the cocoa powder has dissolved. Set it to one side to cool for 10 minutes.

Into a medium bowl, sift then whisk together the remaining dry ingredients.

Into a tall jug, add the eggs, yoghurt and oil. Blend to combine with a hand blender (or whisk by hand in a medium bowl).

Pour the wet ingredients over the dry, and whisk to combine.

Pour the cooled cocoa mixture over the top, and whisk by hand to combine. Pour the batter into the lined cake tin and bake in the centre of the oven for around 35 minutes, or until skewer comes out clean.

Once baked, remove it from the oven and allow it to cool in the tin for a few minutes before flipping it out onto a wire rack. The sponge is very fragile so be careful. Flip the cake from the wire rack, onto some cling film, wrap it tightly and refrigerate for 3 hours (or overnight).

Once chilled, use a serrated knife to level the top of the cake. For this recipe, I am using a slightly smaller ring mold (20cm) for a neat finish to the cake, but you could also build this in a 20cm spring form cake tin, trimming down the cake and then lining the tin with a strip of acetate or baking paper along the sides. This will give you a smooth finish and make it easy to release later.

Place the sponge onto a tray lined with a silicon baking mat, and then press the ring mold around the sponge, removing the excess. Set this to one side.

Into a small bowl, add the powdered gelatin and cold water. Stir this together to combine then allow it to sit at room temperature for 10 minutes. If using leaf gelatin, just soak 1 sheet in a bowl of ice water.

Add the chopped milk chocolate into a tall measuring jug.

Into a medium saucepan, add the milk, 60g of cream, egg yolks and sugar.

Place it over a medium heat, and whisk constantly until it reaches 82-84C (180-183F) on a digital thermometer.

Immediately remove it from the heat and pour it straight over the chocolate. Scoop in the bloomed gelatin (or squeeze the excess water from the leaf gelatin and add this in) and let the mixture sit for 2 minutes.

Blend it till smooth with a hand blender, then transfer it into a medium bowl.

In a separate bowl, whisk the remaining 315g of cream until it forms a medium peak. Fold this through the chocolate base in three parts, whisk a hand whisk, until smooth. Finally, stir through a pinch of flaky sea salt.

Pour the mousse over the chocolate sponge, then refrigerate it for 4-6 hours, or overnight.

To remove the ring mold, gently heat the sides with a hair dryer or heat gun and carefully lift it off. Allow it to sit at room temperature for 20-30 minutes before slicing and serving.