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.  Store in an airtight container for up to 2 days.