Warm oven to 200 degrees centigrade.
Sieve the flours together.
Grease the inside of the dish with butter.
Keep the butter well cooled in the fridge. (You can cut it into 1cm cubes in advance if you like.)
Mix the flours in a bowl and add the butter and salt. Mix with a cooking card or knife, cutting the butter.
When the butter is the size of small beans, use your finger tips to rub it into the flour until it becomes like breadcrumbs. (If the butter starts to melt, place the bowl in ice water to cool down and then continue.)
Add the ice water (not the ice) a little at a time while mixing with a knife until the dough starts to come together. When it does, form it into a ball and wrap with clingfilm and leave to rest in the fridge for 30 minutes.