Line a half baking sheet with parchment paper. Set aside.
Place milk chocolate chips in a medium sized microwave safe bowl and microwave in increments of 15 seconds at 50% power until completely melted and smooth. Stir the chocolate in between each interval.
Pour the chocolate onto the prepared sheet pan and spread evenly with an offset spatula.
Gently press the pretzels into the top of the melted chocolate. The pretzels will not be covered by the chocolate - you will still see them. Use smaller pieces to fill in gaps.
Place the sheet pan into the fridge to chill for about 15 minutes or until the chocolate has set up.
When the chocolate is nearly set up, melt the white chocolate chips just as you did the milk chocolate chips. Place white chocolate chips in a medium sized microwave safe bowl and microwave in increments of 15 seconds at 50% power until completely melted and smooth. Stir the chocolate in between each interval.
Carefully pour the melted white chocolate over the set milk chocolate and pretzel layer. Try to pour as evenly as possible, only using an offset spatula or spoon to spread the chocolate if necessary.
Working quickly, top the white chocolate layer with M&Ms, crushed candy canes, peanuts and sprinkles. All of these toppings are optional.
Place sheet pan in the refrigerator for an hour or until the bark has firmly set up. Break or cut the bark into pieces to serve.