Important: Have all of your ingredients handy before you start making the pralines. Once you start the process of making pralines, it goes quickly. This technique is known as Mise en Place.
Butter a large sheet of wax paper or parchment paper; set aside. You could also use a large silpad.
Spread the pecans out in a single layer on a sheet pan. Roast in a 350 degree oven for 8 to 10 minutes, or until toasted. Set aside to cool and until ready to use.
In a large heavy pan over medium heat, combine sugar, brown sugar, and evaporated milk; cook, stirring constantly until the thermometer reaches an internal temperature of 235 degrees F. or when a small amount of sugar mixture dropped into very cold water separates into hard but not brittle threads.
As soon as the internal temperature of the candy mixture reaches 235 degrees F. on your cooking thermometer, add the butter and vanilla extract. Stir until the butter is fully melted and the mixture is well combined (about 1 minute). Immediately remove the mixture from heat; set saucepan in a large pan of cold water to cool.
This is the type of cooking thermometer that I prefer and use in my cooking. I get many readers asking what cooking thermometer that I prefer and use in my cooking and baking. I, personally, use the Thermapen Thermometer shown in the photo on the right.
When sugar mixture has almost cooled, beat with a spoon 1 minute or until it begins to lose it gloss. Immediately stir in toasted pecan halves.
Drop by tablespoonfuls onto prepared buttered wax paper, leaving about 3 inches between each ball for the pralines to spread. NOTE: Work quickly before mixture sets. If it thickens up, just place pan back on low heat to re-soften.
Let cool until the pralines are firm, approximately 10 to 15 minutes.
When pralines have cooled and have become firm, wrap individually in aluminum foil or plastic wrap and store in a covered container.