Line a baking sheet with foil and grease with butter. Grease a small spoon for scooping the pralines and set aside with the baking sheet.
Add the 4 tablespoons butter, light brown sugar, granulated sugar, salt, heavy cream and corn syrup to a 2 quart or larger heavy bottom sauce pot with a candy thermometer attached and bring to a boil over high heat.
Whisk the mixture to dissolve the sugar then allow it to continue to cook until it reaches firm ball stage, 246 degrees °F, for about 3 minutes.
Remove from the heat and allow it to cool, undisturbed, for 4 minutes.
Add the pecans and vanilla to the pot and using a wooden spoon, beat the mixture vigorously until thickened and the nuts are suspended, about 2 minutes. Working as quickly as you can, portion the pralines with the greased spoon onto the prepared pan. Allow the pralines to cool completely before serving.