Combine Half-and-Half, butter, brown sugar, and granulated sugar in a medium saucepan.
Cook over medium heat, stirring constantly until sugar dissolves and mixture comes to a boil. *Note: if sugar gets on the side of the pan, get a small bowl of water and a pastry brush and wipe down the sides.
Add the candy thermometer (bulb of thermometer should not touch the bottom of the saucepan). Continue to cook until mixture reaches the soft-ball stage (238 degrees F).
Pour into large heat-proof mixer bowl.
Cool until lukewarm (about 110 degrees F). Don't stir it!
Once it gets to the right temperature, add vanilla and beat with electric mixer until thick and candy looses it's gloss.
Beat in pecans.
Spread into prepared pan and refrigerate until firm.