You are about to stop reading barbecue sauce labels. Make this once, put it on the next thing that comes off the heat, and wait for somebody to ask where you bought it.
Prep: 5 minutes
Add everything except the butter to a small saucepan. Set over medium-low heat and stir until the brown sugar dissolves and the mixture looks uniform, with no streaks of ketchup left.
Simmer Low
Cook, stirring often, until the sauce bubbles gently at the edges, 3 to 5 minutes. Do not let it reach a hard boil. Simmer 3 to 5 minutes more, until it coats the back of a spoon in a thin even sheet and a line drawn through it holds for a second before closing.
Finish Off the Heat
Pull the pan off the burner and stir in the butter until it disappears completely and the surface turns glossy. This is what makes it shine; do it over heat and it separates.
Taste and Jar
Add more vinegar if it is sweet, more salt if it is flat, more cayenne if it isn't hot enough for ya. Use the sauce right away while it's warm, or cool completely and pour into a jar. It thickens noticeably as it cools.
Notes
Butter goes in OFF the heat. In a simmering pan, it melts into grease and separates. Off the heat, it emulsifies, and that is where the shine and the round finish come from.
Medium-low is not caution; it is the recipe. Apple butter and brown sugar are both concentrated sugar. A hard boil scorches the bottom of the pan before the top of the sauce knows anything happened, and a scorched batch is bitter and cannot be saved.
Taste at the END, never the start. Apple butters vary wildly, from barely sweetened to basically spiced pie filling. Vinegar and cayenne are your two dials once you know which jar you bought.
Glaze vs mop: straight out of the pan, it is a glaze for the last minutes of a cook. A splash of cider or water loosens it into a mop you can brush on repeatedly over a long smoke.
Brush it on LATE. Every sugar in here burns well before the meat finishes cooking. Last 15 to 20 minutes of a roast or smoke, or toss cooked food in it and give it a short blast of direct heat.
Scaling: doubles and triples cleanly in a larger pan. Add 2 to 3 minutes of simmer per doubling, and judge by the spoon, not the clock.
Storage: airtight jar, 1 week refrigerated, or 3 months frozen (jar with headspace, or an ice cube tray for single portions). Whisk after thawing to bring it back together.
DO NOT water bath can this. It has not been tested for shelf-stable canning, and the acidity has not been measured. Refrigerator sauce only.