Preheat the oven to 350 degrees F. Coat two 9-inch-round cake pans with cooking spray and line the bottoms with parchment paper.

Whisk the cocoa powder and 1 1/2 cups boiling water in a medium bowl until smooth; set aside.

Whisk the flour, sugar, baking powder, baking soda and salt in a large bowl until combined. Add the eggs, vegetable oil, sour cream and vanilla and beat with a mixer on medium speed until smooth, about 1 minute.

Reduce the mixer speed to low; beat in the cocoa mixture in a steady stream until just combined, then finish mixing with a rubber spatula. (The batter will be a thin consistency.)

Divide the batter between the prepared pans and tap the pans against the counter to help the batter settle.

Bake until a toothpick inserted into the middle comes out clean, 30 to 40 minutes. Transfer to racks and let cool 10 minutes, then run a knife around the edge of the pans and turn the cakes out onto the racks to cool completely.

Remove the parchment. Cut each cake into 8 slices, and stick a cake pop stick or candy cane into the bottom if desired.

Mix the food colouring and icing sugar with enough water to make an icing that is a bit runny, but still quite stiff. Try drizzling a bit on a spare piece of paper; you want it to stay in strips, not run all over the place.

Spoon some icing over each cake wedge (you can cover it completely or drizzle lines across them in a tree shape). Decorate with sprinkles, Smarties, M&M, whatever your heart desires and then lift onto a wire rack and leave to set completely (this will take a few hours).