Combine the sugars, milk and butter in a large pot and cook over a medium-high heat, stirring occasionally.
When the mixture comes to a boil, start stirring constantly. Add the pecans. Let it boil for 3 minutes, until the syrup registers 238-240 degrees F on a candy thermometer.
Remove the pan from the heat, stir in the vanilla and keep stirring. The syrup will become creamy and cloudy and will start to thicken.
Drop by spoonfuls onto parchment paper. Let cool.
Notes
To make chocolate pralines - Add 1/2 cup chocolate chips with all the ingredients in the pot.