Preheat oven to 350 °F
In a medium-sized bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt until combined. Set aside.
In a separate large bowl, using either a handheld or stand mixer, combine the butter and sugars, creaming them together on medium speed until light and fluffy (this will take about 3 minutes).
One at a time, add the eggs to the butter and sugar mixture, beating each time until fully incorporated.
Beat in the vanilla.
Using a fine mesh sieve or a sift, sift the cocoa powder into the large bowl with the wet ingredients. Mix until all cocoa powder is incorporated.
Add the flour mixture to the wet ingredients. Mix until the flour is almost fully combined except for a few streaks of flour. At this point, add in your chocolate (either chopped up bars or chips) and the broken pretzel bits.
Scoop out tablespoon-sized cookie dough balls (for cookies as big as the ones in my photos, use three tablespoon portions for one dough ball).
Prepare a baking sheet by lining it with parchment paper. for small cookies, about 12 can fit on a pan at a time with adequate spacing. For larger cookies, about 6 can fit with adequate spacing to leave room for spreading. Bake small cookies for 10-12 minutes, bake large cookies upwards of 20 minutes.
Allow to cool for 5-10 minutes before indulging!