Prepare a baking sheet: Line it with aluminum foil and spray with nonstick cooking spray (or use a silicone mat).
In a medium saucepan over medium heat, combine white sugar, brown sugar, and evaporated milk. Stir until the sugar dissolves completely.
Insert a candy thermometer into the mixture. Cook, stirring occasionally, until it reaches 240°F (soft-ball stage).
Remove from heat. Drop cubes of butter on top — don’t stir yet. Let it sit for 1 minute.
Add vanilla extract and pecans. Stir continuously with a wooden spoon until the mixture thickens and lightens in color.
Once it starts holding its shape (but is still easy to stir), quickly drop spoonfuls of the mixture onto the prepared baking sheet.
Work fast — pralines set quickly in the pan. If the mixture stiffens too soon, stir in 1 spoonful of boiling hot water to loosen it.
Let pralines set at room temperature for 30 minutes, or until fully hardened.
