PREP: Preheat the oven to 325 degrees F. Line an 8 x 8-inch pan with parchment paper (do not skip, these bars are sticky!) and set aside.
COOKIE BASE: In a large bowl, using a hand mixer, cream the room-temperature butter (not melted) with the brown sugar and white sugar. Beat in the egg and vanilla extract. Then add the baking soda, salt, flour, and chocolate chips. Beat until the dough is combined and smooth. It will be a bit wet.
BAKE: Press the dough evenly into the bottom of the prepared pan and bake for 19-23 minutes or until lightly golden brown at the edges and firm in the center (shouldn't be glossy).
MARSHMALLOWS: Remove from the oven and top with the marshmallows. Bake for 1 and 1/2 more minutes and remove from the oven to a cooling rack.
CANDY: Top with the miniature M&M's, coarsely chopped chocolate pretzels, and sprinkles.
COOL: Let Funfetti Bars cool at room temperature for at least an hour, then cut into bars (with a hot sharp knife -- run under hot water, dry, cut, repeat), and enjoy. These are best enjoyed within the same day these are made because of the marshmallows on top.
STORAGE: If you have some leftovers, you can keep these bars, tightly covered, at room temperature for up to 3 days. They can also be stored in an airtight container in the fridge for up to a week, but the marshmallow does lose flavor and texture.
Note 1: Miniature M&M's can be found on the baking aisle of most grocery stores; they're labeled as M&M milk chocolate baking bits.