I cheat and use a food processor to get a uniform crumb but you could rub the butter into the flour mix to make fine breadcrumbs. Put the flour mix into the processor. Add the butter. Process until the butter is well incorporated.
Pour back into the original bowl. Break the egg into the flour mix and add the tablespoon of cold water.
Using a knife, stir the mixture until it starts to clump together and distributes the egg and water.
Bring the mix together with your hand into a ball. Do not knead.