Preheat the oven to 400°F (200°C). Line a rimmed baking sheet with parchment paper.
Arrange the saltine crackers in a single layer, covering the baking sheet as evenly as possible.
In a medium saucepan, melt the butter and brown sugar over medium heat. Bring to a steady boil and cook for 3 minutes, stirring carefully.
Remove the saucepan from the heat and stir in the vanilla extract and optional sea salt.
Pour the hot toffee mixture over the crackers and spread evenly. Bake for 5 minutes, until bubbling.
Remove the tray carefully from the oven. Sprinkle the chocolate chips over the hot toffee and let them sit for 3–5 minutes.
Spread the softened chocolate into an even layer. Add chopped pecans, Christmas sprinkles, or flaky sea salt while the chocolate is still soft.
Allow the toffee to cool, then refrigerate for about 1 hour, or until completely firm.
Lift the toffee from the tray and break it into pieces. Serve or store in an airtight container.