Preheat the oven to 356º F (180° C). Line a rectangular baking pan of approximately 17,80 x 13 inches (45 x 33cm) with parchment paper.
In a medium bowl, sift the flour, salt, baking powder and baking soda.
In another large bowl, beat with an electric mixer or a ballohand whisk 6 tablespoons of butter (90 gr) with the two types of sugar for 3 minutes until the mixture is lighter in color and fluffier.
Add the egg with the vanilla extract. Beat for 1 more minute until integrated.
Add in the flour mixture and beat just 30 seconds until smooth. Finally, add the oats and beat until everything is well combined.
Pour the cookie mixture into the pan and spread with a rubber spatula or your hands until it is about 7 to 8 mm thick. The mixture does not have to be uniform in the pan.
Bake for 13-15 minutes until the cookie is lightly browned. Once ready, remove it from the oven and let it cool for 10 minutes.
For the filling:
In a large bowl, add the 2 types of sugar with the milk powder and salt. Mix with a hand whisk for a few seconds.
Add the melted butter and beat for 1 minute. Then add the whipping cream with the vanilla extract and beat for 2 more minutes until smooth and homogeneous.
Finally, add in the egg yolks one by one, beating for 15 seconds after each addition.
To assemble the pie:
Once the cookie is cool, cut it with your hands into uneven pieces, place it in a food processor with the remaining 3 tablespoons (45 g) of butter and grind until you obtain a sandy and slightly lumpy dough.
Place this dough in a 9 inches (23 cm) diameter pie pan and flatten with your hands until a crust is formed.
Carefully pour the filling on top of the pie base and bake at 356ºF (180°C) for 15 minutes. After this time, lower the oven temperature to 320ºF (160°C) and continue baking the pie for another 15 minutes. In the end, the pie should be golden brown around the edges but soft in the center. If it looks too soft, give it 5 more minutes of baking time.
When the pie is ready, remove it from the oven and let it cool at room temperature for 30 minutes. Then refrigerate for at least 2 hours or overnight before eating.
To serve the pie, decorate it with powdered sugar and it's ready.