Preheat the oven to 160C Fan and grease and line a 9-inch/23 cm loose-based round tin with non-stick baking paper.
In a large bowl, or a stand mixer fitted with the whisk attachment, whisk together the softened butter with the sugar for about 3-5 minutes until pale, light and fluffy.
Add the chocolate and hazelnut spread and vanilla extract and whisk again to combine.
Add the eggs, one at a time, whisking between each addition until you have a smooth batter.
Sieve in the flour, baking powder, cocoa powder and salt in a separate bowl, then gently fold together until you have a spoon batter.
Tip half the mixture into the prepared tin and even out the surface. Arrange half the sliced pears on top.
Pour over the remaining cake batter, smooth over the top and arrange with the remaining pears, fanning out the slices for a decorative effect. Sprinkle over the demerara sugar.
Bake in the oven for about 30-35 minutes until well-risen and just firm to touch. Leave to cool in the tin for at least 15 minutes before turning out onto a wire rack and leaving to cool completely.
Meanwhile, make the chocolate sauce (if using): put the chopped chocolate, double cream and sugar into a medium-sized heat-proof bowl. Set over the pan of barely simmering water, ensuring the bottom of the bowl doesn’t come into contact with the water (or the chocolate will overheat). Melt the mix, stirring often (alternatively, pop into a microwave-safe bowl and melt in 30 second bursts, stirring between each), until smooth.
Dust the cake with icing sugar and serve slices with the hot chocolate sauce and some lightly whipped cream on the side.