To make the oat cookies, pre-heat the oven to 180°c and line a baking sheet with baking paper.
Sift the flour baking powder and salt.
In a separate bowl, cream the butter and sugars until light and fluffy.
Add the egg and mix well.
Add the flour and mix through before stirring in the oats.
Spread into the prepared baking sheet and bake for 20 minutes until golden brown and set.
Remove from the oven and allow to cool.
When the cookies are completely cooled, blend them into fine crumbs in a food processor. Add the butter and sugar and blend until the mixture resembled damp sand.
Press the cookie mixture into the bottom of a tart case and set aside.
Pre-heat the oven to 200°c.
To make the filling, whisk the sugars, salt and milk powder together.
Whisk in the melted butter then the cream and vanilla.
Carefully whisk in the egg yolks, taking care not to incorporate any air into the mixture.
Pour the filling into the prepare shell and bake for 10 minutes until the edges of the filling has set but the centre is still slightly jiggly.
Remove from the oven and allow to cool completely before placing in the fridge.
When you are ready to serve, dust over some icing sugar and slice.