Preheat the oven to 190 C (375 F) and line a 9 x 13 inch tray with baking paper.
Cream the butter, brown sugar and white sugar until light and fluffy. Add the egg and beat well until incorporated.
Stir in the flour, baking powder, soda and salt until just combined. Mix in the oats.
Spread evenly into the prepared tray and bake for 20 minutes, until golden brown.
Allow the cookie to cool completely, then crumble finely.
FOR THE COOKIE BASE
Grease 2 x 12 hole muffin tins well. Do yourself a favour and don't line the tins with cupcake liners. I learned the hard way.
In a stand mixer, food processor or by hand, combine the crumbled cookie, melted butter, brown sugar and salt.
Divide the crust evenly between the 24 muffin holes, about a ¼ cup per hole.
Press down into the base and sides to line each hole with an even layer of the cookie crust.
Set aside as you prepare your filling.
FOR THE FILLING
Preheat the oven to 180 C (350 F).
Mix together the sugars, salt and milk powder.
Whisk in the melted butter, cream and vanilla.
Whisk in the egg yolks gently.
Divide the mixture between the 24 mini crusts. Do not fill them right up to the top. I did and they kind of exploded a little. You may have a little filling left over.
Bake the pies for 15 minutes, then reduce the heat to 170 C (325 F) and bake for a further 10 minutes or so, until the filling is golden brown and slightly wobbly.
Allow the mini pies to cool for a few minutes before removing them from the muffin tins and letting them cool on a wire rack.
Makes 24 mini crack pies. Recipe can be halved.
Recipe by Butter Baking at https://www.butterbaking.com/2012/05/22/mini-crack-pies/