Preheat the oven to 180°C / fan 160°C / gas 4. Grease 2 x 20cm round cake tins, then line with baking parchment. In a small bowl whisk the cocoa powder into 125ml boiling water. Stir in the sunflower oil, then set aside to cool slightly.
In a large mixing bowl, use an electric hand whisk to cream together the butter, sugar and vanilla extract until light and fluffy.
Beat in the eggs one at a time, adding a little of the flour with each egg to stop the mixture splitting. Beat in the cocoa mixture.
Sift over the remaining flour and baking powder, then gently fold in with a wooden spoon until the mixture is smooth.
Divide the mixture equally between the prepared tins and bake in the oven for 40-45 minutes until cooked. The cake is cooked when a skewer inserted into the middle comes out clean. It should be risen and spring back to the touch. Turn out onto a wire rack and allow to cool completely before decorating.
In a large bowl, use an electric hand whisk to beat together the butter and vanilla extract for 2-3 minutes, until pale and soft.
Add half the icing sugar and beat on a low setting at first, so the icing sugar doesn't puff up and out of the bowl. Beat on a higher setting until smooth, then repeat with the rest of the icing sugar. Continue to beat for 1-2 minutes, until pale and fluffy.
Add the melted chocolate and stir until an even colour is achieved.
To make the truffle mixture for the chocolates on top of the cake, gently warm the cream and butter together in a small pan over a low heat. Once the mixture is just steaming, remove from the heat. Break up the chocolate and stir into the hot cream mixture, until melted and smooth. Pour into a small freezer-proof glass bowl or container. Freeze for 1 hour, until solid.
Sandwich the risen sides of the cakes together with a quarter of the buttercream so you have a flat base and a flat top. Place on a chopping board and spread all but 2 tbsp of the remaining buttercream over the top and sides of the cake.
Dust a surface with icing sugar. Roll out 200g of the chocolate icing to a rough long, thin rectangle about 3mm thick. Trim with a sharp knife to a 1.5cm x 62cm strip. Position around the edge of the top surface of the cake to make a lip. Use the remaining buttercream to fill in any gaps between the cake and the icing strip.
Re-roll the icing trimmings and the remaining 800g chocolate icing to a large 4mm-thick circle. It needs to be large enough to cover the cake and sides. Use a rolling pin to drape the icing over the cake. Smooth it down with the palms of your hands and run a finger around the inside of the raised lip to push the icing into place. Trim the excess icing from the base of the cake with a sharp knife.
Lay one of the cake ribbons across the middle of the cake plate. Carefully transfer the cake onto the cake plate and ribbon using two cake lifters or palette knives.
Roll out the pink icing to a rough rectangle about 3mm thick, then cut to a 12cm x 30cm rectangle. Use a ruler and a sharp knife to cut the rectangle into 12cm strips of varying widths: 5mm, 2cm and 2.5cm. Brush each strip with a little water and carefully stick onto the side of the cake, gently pressing over the raised icing lip. Trim with a sharp knife. Brush the chocolate coffee beans with a little water, then secure around the base of the cake to add a neat trim.
Take the frozen truffle mixture out of the freezer and let stand for 5-10 minutes. Use the melon baller to scoop out little balls of the truffle mixture. If you're using a 1/2 tbsp metal measuring spoon, scoop out truffle halves, level with a knife and use a teaspoon to help release the halves, then press two together to make a ball.
Put the different sprinkles in separate shallow bowls, then toss the truffles in the different coloured sprinkles to coat. Put the truffles and chocolate seashells in petits fours cases and place on top of the cake. Carefully pull up the sides of the ribbon and secure with a small knot. Tie the second ribbon in a bow, trim and secure on top of the knot with double-sided sticky tape.