Preheat oven to 350°F (175°C)
Grease a 9×13-inch baking pan and optionally line with parchment for easy removal.
Chop caramels into small pieces.
Break pretzels into small chunks until you have ½ cup.
In a medium bowl, whisk melted butter, brown sugar, and granulated sugar.
Add the egg, extra egg yolk, and vanilla; mix well.
Stir in baking soda, baking powder, and salt.
Add 2 cups flour and mix until just combined. If the dough is very loose, let it rest a few minutes and (if needed) add up to ¼ cup more flour.
Fold in all mix-ins (chocolate chips, M&M’s, pretzels, caramel, sprinkles).
Press dough evenly into the prepared pan.
Press extra mix-ins on top for a festive look (optional).
Bake for 16–18 minutes, or until edges are lightly golden. Do not overbake — bars should remain soft and chewy.
Let bars cool completely in the pan.
For clean slices, chill for 30 minutes, then cut into 12 large, 24 medium, or 48 bite-sized bars.
Wipe the knife between cuts or warm slightly for cleaner edges.
