Preheat the oven to 350°F (180°C), or 160°C for a fan oven. Line 5 baking sheets with parchment paper and set aside.
In a large bowl, whisk together the flour, salt, cream of tartar, baking powder, and baking soda. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, granulated sugar and powdered sugar until light and fluffy, about 3-4 minutes. Add the canola oil and beat again for 2-3 minutes.
Add the eggs and vanilla extract to the butter mixture and beat until fully mixed in.
Add the flour mixture to the butter mixture in 2-3 smaller additions, mixing well each time.
Use a small cookie scoop to drop heaping tablespoons of dough 2-3 inches apart onto the prepared baking sheets. Bake in the preheated oven for 8-10 minutes or until lightly golden and puffed up.
Allow to cool 5 minutes before transferring to a wire rack to cool completely.
Notes
Ingredients at room temperature, including the eggs and butter, will mix together smoothly and give you the best results.
Mix well, but don't overmix. You should add the flour to the bowl in a few smaller additions, mixing after each one until you can't see any more dry streaks.
Cool on the pan for about 5 minutes before moving the cookies to a cooling rack. If you try to move them too soon, they will break.
Adjust if needed. This recipe makes 5 dozen 3-inch cookies. You can easily halve or double the recipe if you need to.
To Store: Keep in an airtight container at room temperature for up to 5 days.