"My Dad's mom would make this for him every time we visited Illinois. As a child, I could hardly wait for the filling to cool enough to eat. The secret is to stir constantly for a rich, velvety smooth pudding-like melt-in-your-mouth experience. These instructions are per my grandmother, written down by my mother not too long before Grandma passed away. Grandma wouldn't give the recipe away until her last years. It's a family treasure now! Top the pie with meringue, whipped cream, or whipped topping."
Whisk sugar, flour, and salt together in a saucepan over medium heat. Add milk and cook, whisking constantly, until mixture thickens and begins to bubble, 3 to 5 minutes. Remove from heat.
Stir pineapple, egg yolks, and butter into milk mixture. Return to heat and cook over medium heat until mixture begins to bubble, 3 to 5 minutes. Continue cooking, stirring constantly, until hot and thick, about 2 minutes more. Pour mixture into prepared pie crust. Refrigerate until chilled, at least 3 hours.