Line a half sheet pan (18x13x1) pan with foil. You want the foil to go up and over the edges of all sides of the pan. Spray well with nonstick cooking spray. Preheat oven to 350 degrees.
Line the bottom of the half sheet pan with one even layer of saltine crackers. This should be approximately 54 crackers. You may have to break a few to cover the bottom of the cookie sheet entirely. Set aside.
In a medium saucepan, heat butter and brown sugar together. Once it's melted, bring to boil, immediately reduce to simmer and time 3 minutes. You want this mixture to get slightly darker as it simmers over the course of 3 minutes.
Remove from heat, stir in vanilla and pour over crackers. Spread mixture over the entire pan of crackers, covering them all as evenly as possible.
Bake 5 minutes and remove from oven. Toffee mixture will look really bubbly and crackers may have shifted slightly. This is normal! Turn oven off.
Top with chocolate chips. Place back into warm oven to melt chocolate chips, 45 seconds to 1 minute more.
Remove from the oven and spread melted chocolate to cover all the crackers evenly.
Sprinkle the top with Maldon sea salt and cool to room temperature. Transfer to refrigerator for 2-3 hours or until completely solid.
Remove from the fridge and peel christmas crack from the foil. There might be some foil stuck to the bottom of the saltine candy as you peel it off. This is normal! Just peel off the missed pieces until all the foil has been removed.
Break or "crack" the candy into small pieces. Place into airtight bags and store in the refrigerator for up to two weeks. Serve cold or at room temperature.