Place 1/3c of unpopped popcorn into a brown paper lunch sack. Fold top of bag over twice and lay fold side down in microwave. Cook for 1:30-2 minutes until popping slows. Repeat with the other 1/3c of popcorn (you can reuse the same brown paper bag).
Pour popped popcorn into a large bowl, allowing the unpopped kernels to fall to the bottom of the bowl. Separate out unpopped kernels.
Pour broken pretzels into the bowl with the popcorn.
In a microwave safe dish, add white chocolate chips. Cook, stirring every 30 seconds, until melted and smooth, being careful not to scorch (mine took about 90 seconds). Pour melted chocolate over popcorn and pretzels. Using a large stirring spoon, stir the chocolate into the popcorn until thoroughly coated. Fold in M&M's and sprinkles.
Spread popcorn out onto wax paper to cool and dry. Once dry (about 10 minutes), break apart. Store in bags or airtight containers.