Preheat your oven to 350°F (175°C). Line a large rimmed baking sheet with parchment paper or a silicone baking mat. This step is crucial to prevent the caramel from sticking to the pan.
Spread the mini pretzels in a single, even layer on the prepared baking sheet. Try to fill in any large gaps so the caramel has a solid base to cling to.
In a medium saucepan over medium heat, melt the butter and brown sugar together. Bring the mixture to a boil. Once it starts bubbling, let it boil for exactly 3 minutes without stirring. Remove from heat and quickly stir in the vanilla extract.
Immediately pour the hot caramel mixture evenly over the pretzels. Use a spatula to spread it to the edges. Place the baking sheet in the oven and bake for 5 minutes until the caramel is bubbly and golden.
Remove the pan from the oven and immediately sprinkle the semi-sweet chocolate chips over the hot caramel. Let them sit for 2 to 3 minutes to soften. Use an offset spatula to spread the melted chocolate into a smooth, even layer.
While the chocolate is still wet, melt the white chocolate chips and drizzle them over the top in a decorative pattern. Sprinkle the flaky sea salt and Christmas sprinkles over the surface.
Let the bark cool at room temperature for 30 minutes, then place it in the refrigerator for at least 1 hour until completely set. Once firm, use your hands or a knife to break the bark into irregular, bite-sized pieces.