Place olive oil, onion, and salt in medium saucepan over medium heat. Cook stirring occasionally, until onions are caramelized.
Add Rub and molasses, stir 30 seconds, then add beer to deglaze pan, whisking well.
Add vinegar, tomato sauce, and brown sugar. Stir and reduce heat to medium-low. Simmer, stirring occasionally, until sauce is reduced and thickened.
When sauce is brown and thick, add espresso and remove from heat.
When cooled, to lukewarm, transfer sauce to blender and cover. Puree on HIGH until smooth, about 1 minute.Use on meat, or keep refrigerated,