Preheat oven to 180 C/ 160 C fan and grease and line three 15 cm cake tins with baking parchment.
Cream together the butter and sugar until and light and fluffy.
Sift together the flour, salt and baking powder in another bowl.
Add the eggs, one at a time, to the butter mixture, along with a spoonful of the flour mixture, beating well after each addition. Add the remaining flour and the sunflower oil. Mix well.
Weigh the batter and divide evenly between two bowls.
Add the peanut butter to one bowl of cake batter, and beat well. Add the cocoa powder to the other bowl and beat well. The batters should be of a soft dropping consistency - add a little milk to thin them, if required.
Bake in the centre of the oven for 30-35 minutes until well rise and golden. A wooden skewer, inserted in the centre of the cake, will come out clean when its ready. Leave to cool in the tins for 15 minutes before transferring to a wire rack to cool completely.
To make the peanut butter frosting beat the ready made vanilla frosting with the peanut butter and milk. Sandwich together the three layers of the cake together with the peanut butter frosting, reserving a little for the spider web decoration on the top of the cake. Cover the sides too with a thin layer of frosting so that you can still see some of the cake edge peeking through. Chill for 15 minutes in the fridge.
Boil a kettle. Break the milk chocolate into a heat proof bowl and add the butter and golden syrup. Pour boiling water into a pan and pop the bowl over the top so that the steam from the water melts the chocolate and butter.
Pour the melted chocolate mixture over the top of the chilled cake, allowing some to drip down the sides. Leave to harden.
Using the remaining peanut butter frosting in a piping bag fitted with a small round tip, pipe a spider web on the top of the cake.
To assemble the spider, cover the Tunnocks Tea Cake in chocolate frosting and then cover in chocolate sprinkles. Repeat with the head, using the black and white hundreds and thousands.
Place the spider parts in the centre of the piped spider web.
Using a piping bag filled with chocolate frosting, pipe eight dollops of chocolate frosting on the back of the spider where you want the legs to attach. Pipe eight corresponding dollops on the cake. Pop the cake into the fridge for a few minutes if you need the icing to harden up a bit, if your kitchen is warm.
Break your chocolate match makers into half, and pipe a small dollop of frosting on the join. Arrange the legs on the spider. Persevere - the legs might want to fall over. Chill your frosting some more if this keeps happening.
Affix the sugar eyes to the spider head with a little chocolate frosting.