Line a large cookie sheet with parchment paper, silicone mat or foil. You can also just spray a cookie sheet with non-stick spray but make sure you cover every inch of it for easy cleanup.
Sprinkle half to ¾ of the nuts on the cookie sheet. Set aside.
Butter a large mixing bowl with butter, all the way up sides.
Place butter, sugar, water and salt in the bowl and cook on high in microwave for 8-½ minutes (based on a 1000-watt oven. Add more for lower wattage ovens) or until candy thermometer reaches 300 degrees (hard crack stage).
Remove from microwave and stir in vanilla. Immediately pour onto prepared cookie sheet covering nuts.
Put chocolate chips on top of the hot toffee and let it sit for 1-2 minutes then spread the chocolate evenly over the surface.
Add remaining chopped nuts and sprinkle with sea salt.
Let cool then place the cookie sheet in the refrigerator for an hour or so to set chocolate.
Take out and break into pieces.
Store in airtight container in cool, dry place.
VARIATIONSNuts - use your favorites as almost any variety works. Try macadamia, mixed nuts, almonds, walnuts or pistachiosChocolate - not into dark chocolate? Try semi-sweet, milk chocolate or even white chocolateOther Toppings - omit the nuts or add dried cranberries, crushed candy canes or pepitas to them.