Preheat the oven to 180°C (160°C fan / 350°F). Grease two 20cm round tins and line the bases with baking paper.

Whisk the flour, baking powder, cinnamon, pumpkin pie spice and salt in a large bowl.

In another bowl, whisk the oil, eggs, brown sugar, caster sugar and pumpkin purée until smooth.

Pour the wet ingredients into the dry and mix just until no streaks of flour remain.

Divide between the tins and bake for 30–35 minutes, until a skewer in the centre comes out with a few moist crumbs. Start checking at 25 minutes.

Cool in the tins for 10 minutes, then turn out onto a wire rack and cool completely.

Chill the sponges for 30–60 minutes, then slice each one in half horizontally to make 4 layers.

Put the sugar and water in a medium saucepan over medium heat. Stir until the sugar dissolves.

Stop stirring once it boils. Simmer, swirling the pan occasionally, until deep amber, about 8–10 minutes.

Take off the heat and stir in the butter. It will bubble up, so be careful.

Slowly pour in the cream, whisking constantly until smooth. Stir in the salt.

Cool to room temperature. It will thicken as it cools.

Beat the softened butter and icing sugar until pale and creamy, about 3 minutes.

Add the cold mascarpone and beat briefly until smooth.

In a separate bowl, whip the cold double cream to soft peaks.

Fold the whipped cream into the mascarpone mixture until thick and pipeable. Don't overbeat or it may split.

Spoon about a third into a piping bag fitted with a round tip.

Place the first sponge layer on a serving board and spread with a thin layer of frosting.

Pipe a border of frosting around the edge, then fill the centre with a generous drizzle of caramel.

Add the next layer and repeat until all 4 layers are stacked.

Finish the top with a piped border, fill with caramel and sprinkle with crushed walnuts, if using.

Chill for at least 30 minutes before slicing.