Combine the granulated sugar, brown sugar, evaporated milk, butter and salt in a heavy saucepan over medium heat.
Cook, stirring constantly, until the butter melts and sugar dissolves, about 4 minutes.
Clip on a candy thermometer and cook, stirring occasionally, until it reaches 235°F (soft-ball stage), 10 to 12 minutes.
Remove from the heat immediately, and stir in the pecans and vanilla.
Beat with a wooden spoon for 2 to 3 minutes, until it turns from glossy to slightly matte and starts to thicken.
Working quickly, drop heaping tablespoons onto the prepared sheets, spaced 1 inch apart.
Let cool and harden at room temperature for 20 to 30 minutes, until firm enough to lift without bending.
Store in an airtight container at room temperature for up to two weeks.
Good to Know
Work fast once it starts to thicken; if it hardens in the pot before you finish spooning, a minute over low heat with a teaspoon of cream loosens it again.