Preheat your oven to 325 degrees F.
In a medium size mixing bowl, whisk the cake flour, dark cocoa powder, and salt to combine. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugars on medium-high until light in color and fluffy in texture, about 3 minutes.
With the mixer on medium speed, add the eggs one at a time, making sure to scrape down the sides and bottom of the bowl between each addition. Continue to beat on medium-high speed for about 2 minutes, until smooth. Add the vanilla and stir to combine.
Turn the mixer to low speed and add 1/3 of the flour mixture to the butter mixture. Mix until just incorporated and then add half of the buttermilk, followed by another 1/3 of the flour mixture, the second half of the buttermilk and lastly, add the remaining flour mixture and mix until just incorporated.
In a small bowl, combine the vinegar and baking soda. The mixture will bubble. Add the mixture to the cake batter and stir on low until combined. Scrape down the sides of the bowl and mix again for another 20 seconds.
Gently fold in the sour cream.
Divide the batter evenly between the prepared pans with the pretzel crust (about 20 ounces in each pan) and bake for about 35 to 40 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs on it. Reminder: we’re baking the cake and pretzel crust together.
Remove the cake pans from the oven and let the layers cool in pans for 15 minutes before inverting onto cooling racks to cool completely.