Prepare & Mix – Preheat the oven to 180°C (160°C fan) or gas mark 4. Grease and line two 20cm round cake tins. In a bowl, whisk together flour, cocoa powder, baking powder, bicarbonate of soda, sugar, and salt, breaking up any lumps.
Combine Wet & Dry Ingredients – In a jug, whisk together the oil and buttermilk, then add vanilla extract and eggs. Pour the wet mixture into the dry ingredients, stirring until fully combined.
Bake to Perfection – Divide the batter between the prepared tins and bake for 25 minutes. Check for doneness with a skewer—if it comes out clean, the cakes are ready. Let them cool in the tins for 15 minutes before transferring to wire racks.
Build the Nest – Line one of the cake tins with oiled baking parchment. Melt the marshmallows and butter in a microwave or on the stove, stirring frequently until smooth. Mix in cocoa powder, crushed pretzels, and shredded wheat. Shape into a nest inside the lined tin and allow it to cool completely.
Create the Icing – Beat the butter, cocoa powder, icing sugar, and milk together until smooth. Add a little extra milk if needed for a creamy consistency.
Assemble the Masterpiece – Layer the cooled sponges with icing in between. Spread icing on top, then carefully place the chocolate nest on the cake. Fill the nest with chocolate eggs and serve. For easier slicing, remove the nest first, cut the cake, and serve the nest pieces separately.