Special equipment: One 10cm Dia. x 6.5cm sphere mold, two 22cm round cake tins, piping bag fitted with a small round tip.
1. Pre-heat the oven to 160°C/320°F. Cover the inside of the half sphere mold with cling film and place the whole thing in the freezer.
2. Butter and flour two 22cm cake tins. Place a circle of greaseproof paper in the bottom of the tins.
3. Beat the butter, with the sugar until pale and fluffy, about 2 minutes. Add the eggs one at a time, beating between each addition. Add the vanilla extract. Mix the flour, baking powder, bicarbonate of soda and salt and add half to the butter and sugar mixture. Add in the milk and beat briefly. Add the rest of the flour mixture and beat again until smooth. Mix in the sprinkles.
4. Divide the cake batter between the two tins and bake for 35 minutes or until a toothpick inserted into the center comes out clean.
5. Allow the cakes to cool in the tins for 10 minutes and then turn out on a wire rack to cool completely. Trim the tops of the cakes for a flat surface if necessary. Split each cake in half.
6. To make the frosting, beat together the butter, chocolate and hazelnut spread, icing sugar, cocoa powder and salt until light and fluffy, about 2 minutes. Add in the milk and beat for a further 2 minutes until smooth and creamy.
7. Place one cake on the board and spread with frosting. Place the other sponge on top and cover more frosting. Top with another sponge and repeat until you have a 4-layer cake. Spread the rest of the cake with frosting. Use a palette knife to smooth the top and sides of the cake.
8. To make the chocolate dome, melt the chocolate pour it into the sides of the mold, using a pastry brush to ensure all the sides are covered in a thin layer of chocolate. Return to the freezer for 20 minutes. Repeat.
9. Pile the top of the cake high with the assorted sweets.
10. Gently pull the cling film to pop the chocolate dome out of the mold. Place the dome on top of the sweets to cover.
11. Decorate with assorted M&Ms. Use melted chocolate to stick them on to the dome.
The person whose birthday it is gets to smash the chocolate dome with a rolling pin and reveal all the sweets before serving.
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