Combine 1/3 of warm oat milk, egg yolks, sugar, vanilla, and cornstarch in a mixing bowl. Mix well until homogenous.

Pour the custard mixture into the remaining warm oat milk, then cook over low-medium heat, stirring constantly with a whisk, until it thickens.

Remove from the heat, then add the cold vegan butter cubes. Divide the custard into two; one is for the chestnut pastry cream, and the other half for the chocolate pastry cream.

Mix half of the hot pastry cream in the chocolate chips, then mix vigorously with wire whisk until the chocolate melts and makes a creamy chocolate custard. Use a hand blender to smoothen out the chocolate pasty cream.

If, for some reason, the pastry cream is not warm enough, melt the chocolate first before adding it to the pastry cream, then mix well until smooth.

Cover with cling film, and then refrigerate for at least three hours.

Make a tube of chestnut cream by wrapping it inside the cling film and shaped into a rod – 23 cm long X 2.5 cm in diameter (9 inch x 1 inch). You can add chopped roasted nuts for a crunchy texture if desired.

Preheat the oven to 170°C (340°F).

Mix and sieve all the dry ingredients in a mixing bowl.

Whip the egg white with the sugar to make a meringue, then set aside.

Add the oat milk to dry ingredients with a whisk or electric mixer until smooth. Fold in the meringue in 3 parts.

Spread the batter about half a cm thick on a baking tray lined with parchment paper and bake for 12-15 minutes.

Let the sponge cake sheet cool down. Cut the dry edges of the sponge sheet before use.

Melt dark chocolate and spread it thinly onto acetate cake strips or parchment paper strips, 2.5 cm wide and 30 cm long (1 in x 12 in).

Place the strips on a slightly curved or flat surface, and then allow the chocolate to harden in the fridge or at a cold room temperature.

Cut the dry edges of the sponge sheet. The sponge should measure 28 cm wide and 38 cm long (11 in x 15 in). Place the sponge sheet on parchment paper, then pipe the chestnut and chocolate pastry creams alternately to cover the sponge on the surface, but keep 1 cm away from the edges.

Place the frozen chestnut rod on the starting rolling edge of the sponge sheet.

Start rolling by keeping some tension in the paper to support the roll along its length, and use your fingers at the ends to help make the first crease. Lift the baking paper upwards, then forward to roll the sponge, using the paper to press gently down on it and keep the roll tight. Keep the seam side facing down.

Chill the rolled log cake in the fridge with the parchment paper on for 30 minutes.

Trim both ends of the log cake to expose the uniform filling. Move to a cake tray or plate. Pipe alternate lines of chestnut and chocolate pastry creams until they cover the entire surface of the rolled cake.

Place the chocolate tree bark strips on the pastry cream lines to cover the log cake entirely. Sprinkle a light layer of icing sugar before serving or cutting.