In a bowl, combine softened butter, brown sugar, and white sugar. Mix until smooth. Add the egg and mix well. Then add flour, rolled oats, salt, baking powder, and baking soda. Mix until you get a sticky dough.
Spread the dough on a baking tray lined with parchment paper. Bake in a preheated oven at 180°C (350°F) for about 25 minutes or until golden brown. Remove and let cool completely. Break the baked dough into pieces and process in a food processor with melted butter and Demerara sugar until you get moist crumbs.
Press the crumbs firmly into a 10-11 inch (26-28 cm) pie dish, forming an even crust. Bake the crust in a preheated oven at 350°F (180°C) for 10 minutes. Remove and set aside.
In a bowl, combine both sugars, add melted butter, and mix well. Add salt, vanilla paste, and heavy cream, whisking together. Finally, add the egg yolks and mix until smooth. Pour the filling into the baked crust.
Bake in a preheated oven at 340°F (170°C) for about 30 minutes. Let the crack pie cool at room temperature, then refrigerate until fully chilled.
Dust with powdered sugar, slice into equal portions, and enjoy!