Preheat the oven to 180ºC, fan 160ºC, gas 4. Grease and line 2 x 20cm round, deep cake tins with baking paper. In a large mixing bowl, use an electric whisk to beat the butter and sugar until light and fluffy.
Beat in the eggs with a little of the flour to stop the mixture from splitting. Then fold in the remaining flour with the vanilla extract, until you have a smooth mixture with no lumps.
Divide the cake mixture equally into the prepared tins. Bake for 1hr 10 minutes, until risen and golden, and a skewer inserted into the centre of the cake comes out clean. Cool in the tin for 10 minutes, then transfer to a wire rack to cool completely.
To make the buttercream, beat the butter and vanilla extract in a large bowl with an electric whisk for 3-4 minutes, until pale and soft. Add in half of the icing sugar and beat on a low setting at first, so the icing sugar doesn't all puff up and out of your bowl. Beat on a higher setting until smooth, then repeat with the other half of the icing sugar. Continue to beat for 1-2 minutes, until the mixture is fluffy and pale.
Use a clean, empty soup or bean tin to press down into the centre of each of the cooled cakes to make a hole. Slice one of the stamped-out circles of cake horizontally and reserve one half.
Sandwich the two main cakes together with a little buttercream and place on the cake board. Fill the middle with the 'surprise' sweets, then place the small reserved cake circle on top to make a lid.
Using a palette knife spread the remaining buttercream over the top and around the outside of the cake. Roll the icing out to a rectangular shape, and cut 4 x long strips of icing approx. 25cm long 2cm wide (2 from each rectangle).
Place the icing strips on the top of the cake in a cross shape, then press lightly onto the sides vertically to secure in place. Re-roll the icing trimmings and cut out two equal-sized diamond shapes. Fold each one in half, then put the two together to form a 'dicky bow'. Cut another small strip of icing and wrap it around around the middle of the bow. Set aside.
Using the flat of your hand, carefully press the 100's and 1000's all over the outside of the cake in between the yellow icing strips. (Place the cake on a lipped tray to prevent them from going everywhere.) Collect any 100's and 1000's that fall off the cake and press them back onto the cake until it is completely covered.
Brush the icing on the cake where the strips cross with a little water. Position the bow on top and press lightly to secure it in place.
Cook's tip: for your leftover sponge, try it instead of swiss roll in our Easy trifle