Line a baking sheet with parchment paper and then spread the pecans in a single layer on the sheet.
In a large skillet over medium-high heat, add the butter and allow it to melt. Once the butter has melted, add the sugar, corn syrup, vanilla, and salt. Mix all the ingredients together and bring to a simmer, continually stirring with a wooden spoon or spatula. Cook until the sugar mixture turns a medium brown caramel color, about 12 to 15 minutes.
Pour the toffee evenly over the pecans. If necessary, smooth out the pecans and toffee using a spatula.
Allow the toffee to cool at room temperature until it is hard and crunchy.
Break into pieces and store in an airtight container.