Preheat your oven to 350°F (175°C). Line a large baking sheet (13x18 inches) with parchment paper, making sure the paper extends up the sides to prevent any sticky toffee from spilling over. Arrange the graham cracker sheets in a single layer on the parchment paper, breaking them as needed to fit snugly and cover the entire bottom of the pan.

In a medium saucepan, combine the melted unsalted butter and packed light brown sugar. Bring the mixture to a boil over medium heat, stirring constantly with a whisk. Once boiling, continue to boil for 3 minutes without stirring. Remove from heat and stir in the vanilla extract.

Carefully and evenly pour the hot toffee mixture over the graham crackers, spreading it gently with a spatula to cover all the crackers.

Bake for 5-7 minutes, or until the toffee is bubbly and slightly darkened. Watch carefully to prevent burning. Remove the baking sheet from the oven.

Immediately sprinkle the semi-sweet chocolate chips evenly over the hot toffee layer. Let them sit for 3-5 minutes to soften. Then, using an offset spatula or the back of a spoon, gently spread the melted chocolate to create an even layer.

While the chocolate is still wet, generously sprinkle with Christmas sprinkles or crushed candy canes. If desired, melt the white chocolate chips in a separate bowl and drizzle them over the bark for an extra decorative touch.

Allow the bark to cool completely at room temperature for at least 2 hours, or place it in the refrigerator for 30-60 minutes to speed up the cooling process. Once firm, lift the parchment paper out of the pan and break the bark into irregular pieces.