Place the flour into the bowl of a food processor, followed by the cold butter and the salt.

Be sure to use cold butter cut into small cubes.

It is thickened with cold butter, whisked in a little at a time.

Cut in the cold butter until the mixture has the texture of coarse meal.

Meanwhile, for the crust, slice the cold butter into a large bowl.

Cut in the cold butter until the mixture has a mealy texture.

Add the cold butter and whisk until the sauce is glossy.

Off the heat, stir in the cold butter until it has emulsified.

The trick is to finish the sauce off just at the last minute with very cold butter or cream.

You probably couldn't cut cold butter with it," Sparrow said.