In a medium bowl, combine yogurt, peanut butter, agave, and vanilla extract and mix until well combined.
Pour yogurt mixture to the prepared baking sheet and spread evenly.
In a small microwave safe container, melt chocolate chips. Add spoonful of melted chocolate on top of the yogurt mixture. Swirl chocolate around with a toothpick.
Top bark with pretzels and chopped nuts and freeze for at least 4 hours to overnight. Serve immediately from freezer.