Preheat your oven to 400°F (200°C).
Line a baking sheet with parchment paper and arrange the salted crackers in a single layer, covering the entire sheet.
In a medium saucepan, melt the butter and brown sugar over medium heat. Stir constantly until the mixture comes to a boil.
Once boiling, continue to cook for 3-4 minutes without stirring, allowing the mixture to thicken and become a rich caramel color.
Remove the saucepan from heat and stir in the vanilla extract if using.
Carefully pour the hot butter mixture evenly over the arranged crackers, spreading it gently with a rubber spatula to cover all crackers.
Place the baking sheet in the preheated oven and bake for 5-6 minutes, or until bubbly and slightly caramelized.
Remove the baking sheet from the oven and immediately sprinkle the chocolate chips on top. Let them sit for 2 minutes to soften.
Using a spatula, spread the melted chocolate into an even layer over the toffee.
If desired, sprinkle the chopped nuts or M&M's over the melted chocolate and press them down gently.
Allow the mixture to cool completely at room temperature, or place in the refrigerator to speed up the cooling process.
Once cooled, break into pieces and serve or package for gifting.