1-Preheat your oven: Begin by preheating your oven to 350°F, as this sets the stage for that perfect bake.

2-Make the oat cookie crust: Cream softened butter with light brown and granulated sugars until fluffy. Add egg yolk and beat until pale, then mix in flour, oats, baking powder, baking soda, and salt to form the dough. Spread the dough 1/4 inch thick on a lined baking sheet and bake for about 15 minutes, but consider underbaking by 10 minutes for easier crumbling later.

3-Cool and crumble the crust: Once cooled, crumble the crust with melted butter and salt until it has a wet sand texture. Press this mixture firmly into a 9-inch pie plate for the best filling-to-crust ratio.

4-Prepare the filling: Combine dry ingredients like sugar and milk powder, then stir in melted butter, heavy cream, and vanilla extract carefully.

5-Adding the Filling and Baking: Add egg yolks to the filling mixture, mixing gently to avoid incorporating air and keep that gooey texture. Pour the filling into the crust until it’s about 3/4 full, then bake at 350°F for 15 minutes until the top turns golden and jiggles slightly. After that, reduce the oven to 325°F and bake for another 5 minutes once the temperature stabilizes.

6-Final baking and cooling: If needed, add 5 more minutes until the center is set but still a bit jiggly, noting that total baking time might reach 31-34 minutes. Cool the pie at room temperature or in the fridge to speed things up, and remember, full cooling is essential for easy slicing. The whole process takes about 2 hours to prep, 25 minutes to cook, 2 hours and 35 minutes to cool, and around 5 hours total.