Preheat the oven to 180° C/ Fan 160°C/Gas 5 . Grease a 10 inch bundt cake tin.
Pour the milk into a small measuring jug and add the lemon juice. Stir once and set aside for 10-15 minutes to curdle. Put the chestnut puree into a large mixing bowl. Add the coconut oil and maple syrup and whisk until smooth.
In a separate bowl, sieve together the flour, baking powder, bicarbonate of soda, cocoa powder and a pinch of salt, then add to the chestnut puree mix and stir. Add the milk and whisk until the batter is smooth and fully combined, then pour in.
Bake for 25 minutes or until a skewer inserted into the middle of the cake comes out clean. Remove from the oven and set aside to cool.
Once the cake has cooled, prepare the ganache. Break the chocolate up into small chunks and place in a small saucepan with the cream. Heat gently over a low heat, folding the cream and chocolate together until smooth and glossy. Do not over stir as it may separate. Set aside to cool slightly, but not set.
Toss the chopped chestnuts together with the sugar.
Pour the ganache evenly over the cake, then top with the chestnuts.