Preheat the oven to 350˚F and prepare three 6-inch cake pans or two 9-inch cake pans by spraying the sides with cooking spray and fitting a parchment or wax paper circle to the bottom of each pan.
In a food processor, grind the salted pretzels into fine crumbs. Keep in mind that 1 1/2 Cups of whole pretzels equals about 2/3 cups of finely ground crumbs. Place the ground pretzels into a medium sized bowl and add the sifted cake flour, baking powder, baking soda, and salt. Whisk to combine and set aside.
In the bowl of your stand mixer (a handheld mixer works fine too!), cream the butter for on high for two minutes until it's light and fluffy. Add in sugar and continue to mix on high for another two minutes, scraping down the bowl and paddle as needed. Add the eggs one at a time, mixing thoroughly after each addition. Add vanilla and sour cream and mix for one minute on high, scraping down the bowl and paddle once more.
With the mixer on low speed, add in the dry ingredients and mix until just combined. Add the milk slowly and mix until just incorporated. Scrape down the sides and bottom of the bowl and give it a few stirs to make sure there are no lumps (without over-mixing). The batter will be slightly thick, but pourable.
Pour batter evenly into prepared cake pans (no more than 2/3 of the way full) and bake for 30-35 minutes. They're done when they spring back to the touch and a toothpick inserted into the middle comes out clean. Let the cakes cool in the pan for five minutes before turning them out onto a wire rack for an additional few hours of cooling. Make sure they're entirely room temperature before applying any frosting.