Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Grease a 23 x 33cm/9 x 13 in rectangular cake tin and line with baking parchment.
Sift the plain flour, cocoa powder, baking powder, bicarbonate of soda and salt into a large bowl then whisk in the ground hazelnuts.
In a large jug, whisk together the milk, oil, yogurt, sugar, lemon juice and vanilla extract.
Pour the wet ingredients into the dry and stir with a balloon whisk until no dry lumps remain.
Pour the batter into the prepared tin and spread it level.
Bake for about 30 minutes until a skewer inserted into the centre comes out clean. Leave to cool completely in the tin.
To make the ganache, place the finely chopped hazelnut chocolate in a heatproof bowl.
Heat the cream until almost simmering then pour it over the chocolate. Leave to sit for 5 minutes then stir until melted and smooth.
Leave to cool until it is firm enough to spread then use a palette knife to spread it over the top of the cooled cake. Decorate the cake with chopped roasted hazelnuts if you like.
Store the cake in an airtight container at room temperature for up to 4 days.