Vanilla Chocolate shortbread

We're having this for our dessert tonight, served with organic ice cream (not homemade although we have made it before). I've gone all American because I'm working out things with cups rather than ounces.

Vanilla Chocolate Shortbread

2 and a 1/2 cups of plain flour (actually a bit less but remove some, about 1 heaped tablespoon, of the plain flour and replace with cornflour to give your shortbread a softer texture)
1 cup caster sugar
2 sticks butter (about a cupful)
2 tbsp cocoa powder (I use Green and Black's)
1 tsp Vanilla essence

1. Sieve flour into bowl, add sugar and then butter.
2. Rub ingredients together - mixture will go crumbly (a bit like for apple crumble topping) and then rub it until it forms a very soft crumbly doughlike consistency (make sure it doesn't go sticky though - it should be 'short')
3. Divide mixture in half
4. To one half, add the cocoa and mix in well
5. To the other half, add the vanilla essence (I just pour it from the bottle in a quick flick) and mix that in
6. Take your baking tin and grease well with butter (I use 2 tins - round Victoria sponge ones)
7. Pat down half the mix and then add the other half on top (or if using 2 tins then do different tops and bottoms - so one is chocolate on the bottom, vanilla on top or vanilla on the bottom with chocolate pressed on top)
8. Bake in the oven for about 10 minutes - with the first bake mark out segments with a knife or slice.
9. Put them back in the oven and bake for a further 15 minutes. Remove and let cool for 10 minutes.

Serve with ice cream...(you may want to serve them warm).

Tip: if you use unsalted butter then you may wish to add a pinch of salt to the mix - it gives them a better flavour.