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Heat the oven to 180C/160C fan/gas 4. Tip the sugar, butter and dark chocolate into a saucepan over a low heat and stir until glossy and melted. Remove the pan from the heat and leave to cool for 10 mins.
Whisk the eggs into the cooled chocolate mixture, then pour into a large bowl with the ground hazelnuts, flour, cocoa and baking powder. Gently stir to combine, then pour into a 20cm tin lined with baking parchment.
Bake for 40-50 mins, or until a skewer inserted into the middle comes out clean. If any wet mixture remains, return it to the oven for 5-10 mins. Leave to cool in the tin for 5 mins, then turn out onto a wire rack to cool completely.
For the ganache, pour the yogurt into a heatproof bowl and set it over a pan of simmering water, making sure the bowl doesn’t touch the water. Whisk for 3-4 mins until hot, then tip in the chocolate. Turn off the heat and whisk continuously until the chocolate has melted. If the mixture looks as though it might split, pour in a splash of milk, leave it for 15 seconds to warm through, then whisk briefly until smooth. Leave the ganache to cool for 15-20 mins so it’s easier to spread.
Spread the ganache all over the cake, then scatter over the roasted hazelnut halves. Leave the cake to cool and set completely. Will keep in an airtight container, chilled or at room temperature, for up to two days. Serve at room temperature.