Melt one tablespoon of butter in 3-quart heavy saucepan. Swirl to coat the bottom of the saucepan.
Cook brown sugar, sugar, half-and-half and remaining butter in the saucepan over low heat, stirring constantly, until the sugars dissolve and the butter melts.
Stir in pecans and cook over medium heat. Allow to boil for 3 minutes, stirring often, until a candy thermometer reaches 238 degrees, the softball stage.
Remove from heat and stir in vanilla. Stir for about 3 minutes until thickened.
Scoop tablespoons of pralines onto a baking sheet lined with waxed or parchment paper. Allow to dry for about 3 more minutes.