The meringue mushrooms can be made up to a week ahead. Heat the oven to 120C/100C fan/gas 1/2 and line a baking tray with baking parchment. Tip the egg whites into a clean bowl and beat with an electric whisk to stiff peaks, about 3 mins. Add the sugar, a spoonful at a time, whisking continuously until all the sugar has been incorporated and the meringue is thick and glossy – it should hold its shape when the beaters are lifted out. Rub a little of the meringue between your fingers to ensure the sugar has dissolved.

Spoon the meringue into a piping bag fitted with a large round nozzle. Pipe the mushroom tops by holding the bag 0.5 cm above the tray and squeezing out squat circles in varying sizes (you should have 12-15). Pipe short lines for the stems, then dab down any peaks using a wet finger. Dust the tops with cocoa and bake for 1 hr until the meringues easily peel away from the parchment. Leave to cool completely in the oven with the door closed. Will keep in an airtight container for up to a week.

Heat the oven to 180C/160C fan/ gas 4 and line the base and sides of a 32 x 22cm swiss roll tin with baking parchment. Beat the egg whites to stiff peaks in a clean bowl using an electric whisk, then beat in half the sugar, a spoonful at a time, until thick and glossy. In a second bowl, beat the yolks with the remaining sugar and the vanilla until pale and fluffy. Add the chocolate hazelnut spread and beat for 1 min more. Beat a third of the whipped egg whites into the chocolate mixture to loosen. Gently fold in the remaining egg white mixture in two batches until just combined. Sift in the flour, cocoa and a pinch of salt, then fold again until no streaks of chocolate remain, but be careful not to over-mix or you will knock out the air. Pour the batter into the tin, spread evenly to the edges and bake for 12-15 mins until set and springy.

Cover the sponge, still in the tin, with a sheet of parchment, then a clean tea towel. Leave to cool completely. For the buttercream, beat the butter until smooth in a large bowl using an electric whisk. Add the sugar, half the milk and the vanilla. Beat for 1 min until creamy.

Invert the sponge onto the baking parchment and tea towel, and lift the tin away. Use a sharp knife to score an indentation about 1cm in from one of the long edges, keeping that side closest to you and being careful not to cut all the way through. Spread over a third of the buttercream, leaving a 1cm border at each of the longer sides. Use the parchment and tea towel to help you roll the sponge into a tight spiral. Trim off a piece about 8cm in length on a slight angle. Transfer the main piece to a serving board and tuck the trimmed part against it to look like a branch.

Mix the cocoa and remaining milk into the buttercream. Spread all over the yule log using a palette knife. Be sure to cover the join well, then use a fork to drag lines through the buttercream for a bark effect.

Sprinkle the hazelnuts and pumpkin seeds over and around the log. Stick the mushroom bases and tops together using the melted dark chocolate, and stick these to the log. Will keep in an airtight container for four days.