Combine Dry Ingredients. In a medium bowl, combine gluten-free flour, salt, baking soda, and baking powder. Whisk to combine well. Set the bowl aside.
Cream Butter & Sugars. In a stand mixer or large mixing bowl, combine softened butter, sugar, and brown sugar. Beat with paddle attachment (or a handheld electric mixer) 2-3 minutes on medium speed to cream the butter and sugar together until smooth, light, and fluffy.
Finish Wet Ingredients. Scrape down the sides of the bowl and mix in the eggs one at a time, fully incorporating each egg before adding the next egg. Then mix in the vanilla.
Add Dry Ingredients. Scrape down the sides of the bowl again, and add the prepared dry ingredients. Mix until almost fully combined, with just a few powdery patches remaining.
Add Mix-Ins. Then, add chocolate chips, pretzel pieces, m&ms, toffee bits, and the sprinkles. Mix gently to evenly distribute the mix-ins throughout the dough.
Chill 30 Minutes & Preheat The Oven. Add the dough to the refrigerator for 30 minutes, while you preheat the oven to 375 degrees. Line two cookie sheets with parchment paper. (This will help control the spread of the cookies.)
Scoop & Roll. Use a large cookie scoop to form 3 Tablespoon balls of cookie dough. Roll each ball smooth between your palms, and place 6 cookies on one of the prepared baking sheets. (Return the rest of the dough to the fridge in between batches)
Bake The Cookies. Place the pan in the oven and bake 9-11 minutes, or until the edges are set and the cookies are just starting to turn golden. (Try not to over-bake. They'll continue to set as they cool.)
Immediately Finish The Cookies While Warm. Right after removing the cookies from the oven, top each cookie with a pretzel and a few extra mix-ins. If desired, use a large round cookie cutter or a spatula to gently shape the cookies into perfect circles. (Some mix-ins can cause slightly wonky spread.)
Cool Completely & Repeat. Allow the cookies to cool 1-2 minutes on the baking sheet before transferring to a wire rack to finish cooling completely. Repeat this process until all the cookies are baked, alternating baking sheets with each batch so you don't put cool dough on a hot baking sheet.
Serve & Store. Enjoy the cookies right away or store for later. Store baked cookies in an airtight container 1-2 days at room temperature, or freeze up to 2 months.