Preheat your oven to 350°F (180° C), then line a cookie sheet with parchment paper and set aside. The number of batches will depend on how large/small you want your cookies.
Combine the flour, cornstarch, baking powder and salt in a medium-sized bowl and set aside.
Cream the butter and sugar together in a large bowl with an electric mixer on medium speed until light in color and fluffy. Add cream cheese and vanilla and mix until well combined.
Add the flour mixture and mix until the dough comes together and is combined. Stir in the M&Ms, chocolate chips and pretzels. Do not over mix.
Drop by rounded tablespoon onto the prepared baking sheet. For evenly sized cookies I like to use a small spring-loaded ice cream scoop to spoon out the dough. Gently press a few more M&Ms, white chocolate chips and pretzels on top of each dough balls if desired (this ensures you can see the chips and the M&Ms after they bake!).
Bake for 8-10 minutes, until very lightly colored on top and around the edges. Allow to cool on baking sheet for 5 minutes, then transfer to a wire rack to cool.