Place chopped pecans into a pan on the stove at medium heat. Stir constantly to avoid burning and toast the pecans until they become a little darker in color and smell strongly of nuts. You can also put the nuts on a sheet pan and toast them in the oven at 350°F/177°C until they take on a little color (approximately 10 minutes). Remove the pecans from the stove AND the pan (nuts can burn easily if left unattended while toasting or in the pan after toasting has completed). Allow nuts to cool completely.
Place a sheet of parchment paper into the bottom of a half-sheet pan. Set a second sheet of parchment paper or a silpat next to the pan.
When making caramel, timing is important so measure out all ingredients and have them prepared ahead of time.
Melt the butter in a medium saucepan (minimum 2-quart size) over medium heat. Gently stir in the sugar, water, and corn syrup using a silicone spatula until the sugar has dissolved. Add the candy thermometer to the pan and increase the heat to high.
Cook the sugar mixture until it reaches 310°F/154°C on a candy thermometer. Stir in the baking soda and salt. Be very careful when adding the baking soda because it will cause the caramel to bubble up vigorously. Do not get burned!
Continue to cook the caramel until it reaches 320°F/160°C. This usually takes seconds, so don’t take your eyes off of it.
Remove the pan from the heat and the candy thermometer from the pan (place it on a heat resistant surface). Quickly stir in the vanilla paste and chopped, toasted pecans using a clean silicone spatula.
Quickly and carefully pour the toffee onto the prepared sheet pan. Use the spatula to roughly spread the toffee. Then, cover the toffee with the second sheet of parchment paper/silpat and use a rolling pin to smooth it out. Do not press too hard while rolling, because you can squeeze the toffee over the side of the parchment paper and burn your fingers. Allow the toffee to cool completely (approximately 4 hours).
This toffee can be stored in an airtight container at room temperature for up to 1 month, although it has never lasted that long in my house! If the toffee is left uncovered in a humid environment, it tends to get sticky and soft.