Preheat the oven to 250°F. Put the pecans in a shallow baking pan or rimmed cookie sheet in a single layer. Bake until aromatic, about 15 minutes. Remove from the oven, stir, and let cool. Set aside.
Combine the evaporated milk, heavy cream, and sugar in a large heavy saucepan over medium heat and stir to mix. Cook, stirring often, until the sugar is completely dissolved. Continue to cook, stirring often, until it comes to a boil. Add the corn syrup and stir constantly (stay with it - and keep stirring to prevent it from boiling over) until it reaches the soft-ball stage, about 240°F on a candy thermometer. (Drop a small spoonful of the boiling sugar syrup in a cup of cold water. Knead the syrup between your index finger and thumb. It should form a soft ball on your finger.) It will take 35 to 45 minutes, depending on the weather.
Add the pecans and stir to mix. Continue to stir over medium heat until the mixture returns to a rolling boil. It will thicken, and as you stir, you will see a film of sugar begin to form on the inside of the pot as it reaches the soft-ball stage again. (Eula Mae has her own rule about when the syrup is ready - when you lift the spoon and two drops meet as they drip off the bowl of the spoon.) Remove from heat.
Add the vanilla and butter and stir to mix. Continue to stir until the mixture becomes thick and creamy, and sugary around the edge of the mixture, about 15 minutes. The mixture will be a taffy color.
Working quickly, spoon the mixture, about a heaping kitchen spoon at a time, onto the waxed paper. Let cool completely before lifting off with a thin-bladed knife. The pralines can be stored between layers of waxed paper in an airtight container for up to 1 week.
Makes about 2 dozen.
It's best to make pralines when the weather is cold and dry, as dampness will delay (or prevent) the candy from setting. "Before you begin, put several layers of old newspaper on your countertop. Then top with a single layer of waxed paper." Recipe from "Eula Mae's Cajun Kitchen Cookbook"