Heat oven to 350°F. Line a rimmed sheet pan with foil and parchment for easy removal.
Arrange crackers in a tight single layer, covering the pan edge to edge.
Melt butter and brown sugar in a saucepan over medium heat, stirring until smooth.
Bring to a steady boil and cook 3 minutes, stirring often, until thick and glossy.
Stir in vanilla, cinnamon (if using), and sea salt. Immediately pour toffee over crackers and spread evenly.
Bake 8–10 minutes until the toffee is bubbling across the surface.
Scatter chocolate chips over hot toffee. Let sit 3 minutes, then spread into a smooth layer.
Top with nuts, a pinch more flaky salt, and sprinkles if desired. Cool, then chill 30 minutes to set.
Lift from the pan and break into pieces. Serve or store airtight.