Special equipment: A candy thermometer
Line an 8- by 8-inch baking pan with parchment paper. In a cast-iron pot with a candy thermometer attached, bring the brown sugar, granulated sugar, butter, evaporated milk and salt to a boil over medium heat, stirring constantly. Keep stirring until the thermometer registers 240 degrees F (the soft-ball stage). Remove from the heat and allow to cool to lukewarm.
Add the maple syrup and vanilla. With a hand mixer on low, mix in the powdered sugar; keep mixing until the white of the powdered sugar is gone. Using a silicone spatula, continue mixing for 5 minutes until the mixture is smooth, creamy and pale. Fold in the pecans. Scrape the mixture into the prepared baking pan. With the silicone spatula, flatten and smooth out the mixture. Lightly dust with cayenne pepper. Set aside and allow to cool for at least 2 hours.