Preheat the oven to 400°F (200°C). Line a 10x15 inch rimmed baking sheet with parchment paper, letting it go up the sides.
Arrange the saltine crackers in a single tight layer on the pan, breaking a few to fill gaps.
Melt the butter and brown sugar in a medium saucepan over medium heat. Stir constantly, bring to a boil, and boil for 3 minutes. Remove from the heat and stir in the vanilla.
Pour the hot toffee evenly over the crackers and spread quickly to cover every cracker.
Bake for 5 to 6 minutes, until bubbling all over. Watch closely so it does not burn.
Remove from the oven and sprinkle the chocolate chips over the top. Wait 3 to 5 minutes, then spread the melted chocolate into an even layer.
Add sprinkles, peppermint or nuts right away while the chocolate is still soft.
Let cool for 15 minutes, then refrigerate for at least 1 hour until firm. Break into pieces and serve.