Preheat your oven to 350°F (177°C) and line two cupcake pans with 21 cupcake liners. *See note below if you'd like to make mini sourdough snickerdoodle cookies to top the cupcakes with.
In a medium bowl, whisk together the flour, baking powder, cinnamon, and salt.
In a large mixing bowl, whisk together the melted butter and sugar until well combined. Add the eggs and vanilla and mix until smooth.
Add the sour cream and sourdough discard and whisk until fully incorporated.
Gradually mix in half of the milk, followed by half of the dry ingredients. Stir to combine, then add the remaining milk and remaining dry ingredients. Mix until just incorporated.
Divide the batter between the cupcake liners, filling each about two-thirds full.
Bake for 20 to 22 minutes, or until a toothpick inserted into the center comes out clean.
Cool cupcakes in the pan for 5 minutes, then transfer to a wire rack to cool completely before frosting.