Heat the oven to 180°C/350°F/gas mark 4. Cream together the butter and sugars in an electric mixer fitted with a paddle or flat beater for two to three minutes, until fluffy and pale yellow.

Add the yolk and beat for a further one to two minutes until the sugar has fully dissolved and the mixture is pale white.

Gently mix in the remaining ingredients and ½ tsp salt until the dough comes together.

Lightly grease and line a 25 x 31cm (10 x12in) baking-tray. Put the cookie dough in the centre of the tray and use a spatula to spread it out to 5mm (¼in) thick. Bake for 15 minutes, or until it is caramelised on top and puffed slightly but set firmly. Cool completely before using.

For the filling, mix the dry ingredients with 1½ tsp salt in an electric mixer. Add the butter and beat on a low speed for two to three minutes until moist. Add the cream and vanilla and mix for two to three minutes. Scrape down the sides of the bowl, add the egg yolks and mix until glossy and homogenous.

To make the pie shells, pulse the oat cookie with the brown sugar and ¼ tsp salt in a food processor until it resembles wet sand. In a bowl, knead the crumbs with the butter until moist enough to form a ball. Divide the mixture between two 25cm (10in) tins, pressing it firmly and evenly into the bottom and up the sides.

Put both pie shells on a baking-tray and divide the filling evenly between them. Bake for 15 minutes until golden brown but still very jiggly.

Open the oven door, reduce the temperature to 170°C/325°F/gas mark 3, and keep the door open with the pies inside until it reaches that temperature. Then close the door and bake for a further five minutes.

Transfer the pies to a rack to cool to room temperature, then freeze for at least three hours, or overnight, for a dense final product.

Defrost for a minimum of an hour before serving cold with a sprinkling of icing sugar.