Mix dry ingredients. In a small bowl add the cinnamon and sugar, stir to combine, and then set aside. In the bowl of your stand mixer (or a large mixing bowl) add the all-purpose flour, whole wheat flour, salt, and vanilla powder (if using vanilla extract, wait until the next step). Stir to combine.
Add sourdough. Add the sourdough starter (and vanilla extract if using) to the flour mixture.
Mix. With the paddle attachment attached, mix the starter into the flour mixture until it’s worked in well. The dough will be slightly crumbly
Add cinnamon sugar mixture. Add 2 tablespoons of the cinnamon sugar mixture to the flour sourdough mixture. Stir to combine.
Add butter. Add 2 tablespoons of the melted unsalted butter and work the butter into the dough until the dough comes together.
Separate and chill. Once a nice dough is formed, turn the dough out onto a clean dry surface. Separate the dough into 2 equal pieces, wrap in plastic wrap, and place it in the fridge for at least 30 minutes up to overnight.
Roll out dough. Preheat the oven to 375℉. Remove 1 wrapped dough from the fridge. Working with 1 dough ball at a time, roll the dough out between 2 pieces of parchment paper (this helps prevent sticking and easy transfer to the baking sheet) until it is about ⅛ inch thick.
Cut dough. Using a pizza cutter, cut the dough into squares, creating cracker shapes.
Prep. Transfer the dough to a lined baking sheet. Using a fork or a toothpick, press into the squares to prevent the dough from puffing up while baking. Brush the tops of the crackers with melted butter. Then sprinkle the cinnamon sugar over top.
Bake and cool. Bake at 375℉ for 20-25 minutes or until the edges are golden brown and they have crisped up. Allow to cool for about 10 minutes before enjoying. Serve with fruit or eat alone.