Peel the peaches by scoring a small X on the bottom of each, dropping them into boiling water for 30 to 40 seconds, then transferring to an ice bath. The skins will slip off easily.
Cut each peach in half, remove the pit, and dice the flesh into rough 1/2-inch pieces. Measure out 4 cups (680 g) of diced peaches.
Add the diced peaches to your medium saucepan along with the ketchup, apple cider vinegar, brown sugar, and molasses. Stir in the Worcestershire sauce and Dijon mustard until everything is combined.
Add the minced garlic, onion powder, smoked paprika, salt, black pepper, and cayenne if using.
Set the pan over medium heat and bring the mixture to a gentle bubble, stirring often. Once it starts bubbling, lower the heat to medium-low and let it simmer uncovered for 20 to 25 minutes, stirring every few minutes and scraping the bottom so the sugars do not stick or scorch.
If using bourbon, stir it in during the last 5 minutes of simmering. Let it cook so the alcohol mellows and leaves behind a warm, oaky background note.
Remove the pan from the heat and let it cool for about 5 minutes so it splatters less. Use an immersion blender to purée the sauce directly in the pot until it reaches your preferred texture.
For a silky, restaurant-style finish, pass the blended sauce through a fine mesh strainer.
Taste the sauce and adjust. Add a pinch of salt for savoriness, a splash of vinegar for tang, or a little brown sugar for sweetness. If it is thicker than you like, whisk in a tablespoon of water at a time until it loosens.
Cool completely, then transfer to clean glass jars and refrigerate.