Line a baking sheet with parchment; spray the parchment with nonstick spray.
Melt the butter in a Dutch oven, then add the sugars, milks and salt. Place a candy thermometer on the side of the pot, bring the mixture to a boil over medium heat and cook until it reaches soft-ball stage or 235 degrees F, about 10 minutes. Whisk in the pecans and cook, stirring constantly, until creamy and starting to thicken, 3 to 5 minutes; if it gets too thick too fast, add a splash of milk to loosen it up. Drop the pralines 1/4 cup at a time onto the prepared baking sheet. Rest at room temperature to cool, about 5 minutes.