Preheat the oven to 350 degrees F.
Line a baking pan with parchment or wax paper.
Place pretzels on the sheet in a single layer.
In a medium saucepan, melt the butter over medium heat. Add the brown sugar and bring to a boil, stirring continuously.
Allow the mixture to boil for 2–3 minutes, bubbling and caramelizing.
Pour over the pretzels and place in the oven to cook for 10 minutes.
Remove from the oven and immediately sprinkle white chocolate chips over the hot toffee.
Spread the melted chocolate over the whole thing with the back of a spoon.
Top with more broken pretzels and Valentine sprinkles, if desired.
Allow the pan to cool to room temperature before placing it in the fridge to set.
After about an hour, remove the cookie sheet from the fridge and break the pretzel toffee bark into bite-sized pieces.