Preheat oven to 425 degrees. Line a 15x10-inch jelly roll pan with heavy duty aluminum foil, and spray with non-stick spray. Arrange the saltine crackers in rows on the pan, lining them up so the edges meet and there is no space between them.

Combine the butter and brown sugar in a large microwave-safe bowl. Heat on HIGH power in 1-minute intervals, stirring after each interval, until sugar is completed dissolved, about 3 minutes total. Stir in the condensed milk. Pour the mixture evenly over the cracker layer. Place the pan into the heated oven and bake for 10-12 minutes, until the butter/sugar layer is brown and bubbly.

Pull the pan out of the oven and place on a cooling rack. While everything is still piping hot, sprinkle the chocolate chips evenly over the cracker layer. Allow everything to stand for about 2 minutes to soften before using a spoon or spatula to spread the chocolate into an even layer over the surface of the bars. Sprinkle the toffee bits in an even layer over the surface of the toffee. If desired, add a pop of festive holiday color by sprinkling on the holiday jimmies.

Allow the toffee bark to cool completely. When cool, break the bark into cracker-sized squares. To store, place in an air-tight container in the refrigerator for up to 5 days.