Preheat the oven to 190C / 170C Fan. Grease and line a 23cm (9-inch) round loose-based cake tin with non-stick baking paper. Set aside.

In a small bowl, whisk together the cocoa powder, boiling water and instant coffee until combined and glossy. Set aside.

In a large bowl, sift in the flour, baking powder, bicarbonate of soda, caster sugar and salt.

Add the egg, yoghurt and the vegetable oil. Give it all a good mix to combine, then stir in the chocolate-coffee mix until you have a smooth, chocolatey cake batter.

Pour the batter into the prepared cake tin and bake for about 20-25 minutes - or until it springs back to touch. 

Set aside to cool in the tins for 5 minutes before turning out onto a rack and leaving to finish cooling completely.

Once the cake has cooled, prepare your cake tin once again. Clean well before greasing and lining with a non-stick baking paper - this time, ensure that the lining on the inside comes at least 2.5cm (1-inch) above the top of the cake tin (a bit like a collar). This is just an insurance policy to ensure you have enough room for the chocolate mousse. Gently slide the cooled cake back into the tin.

Next, make the chocolate mousse: place the chopped chocolate into a heat-proof bowl. Set over a pan of barely simmering water, ensuring the bottom of the bowl doesn’t come into contact with the water (or the chocolate will overheat) and allow it to slowly melt, stirring often until smooth. Alternatively, pop into a microwave-safe bowl and melt in 20 second bursts, stirring between each. This is the quickest method but you have to watch closely and ensure it doesn’t overheat or it will burn and you will have to start again. Once melted, set aside to cool slightly.

Meanwhile, whisk the cream until you have soft peaks - take care not to over whisk. Gently fold in the cooled melted chocolate until no streaks remain. 

Spoon the chocolate mousse mix over the cake and spread in a smooth, even layer. Cover with cling film and chill in the fridge for at least 4 hours — or overnight — until set.

Once the mousse has set, remove from the tin, peel off the baking parchment and set onto a flat serving plate.

Next, make the chocolate ganache: pop the chopped chocolate into a heat-proof bowl and set aside. Pour the cream into a small saucepan and set over a low-medium heat. Slowly bring to a simmer, then remove from the heat and pour over the chopped chocolate. Allow to sit for 2 minutes before gently stirring to encourage it to melt. Continue to stir until glossy and smooth.

Pour the ganache over the centre of the cake, then use the back of a spoon or palette knife  to gently spread it over the top, allowing it to drip down the sides. 

Chill in the fridge for 30 minutes to allow the ganache to set. 

To serve: either serve slices as is, or top with piped whipped cream (I use a star nozzle), seasonal berries and edible flowers. You can simply spoon the whipped cream if you prefer not to pipe.