Preheat the oven to 350°F. Line a rimmed baking sheet with foil or parchment paper and arrange the graham crackers in a single layer so the edges are touching.
In a medium saucepan over medium heat, melt the butter and brown sugar together. Bring the mixture to a boil, stirring often, and cook for 3 minutes until thick, bubbly, and caramel-like.
Carefully pour the hot toffee mixture over the graham crackers, spreading it out evenly with a spatula. Bake for 5 minutes.
Remove from the oven and immediately sprinkle the white chocolate chips on top. Let them sit for 1–2 minutes to soften, then spread into a smooth layer.
Scatter the chopped Oreos and festive sprinkles over the melted chocolate, gently pressing them in so they stick. Add any Halloween decorations while the chocolate is still soft.
Chill in the fridge until completely set. Break into pieces and serve!