The gold standard of all breakfast tacos in Texas, from Valentina’s Tex Mex BBQ in Austin. (Photo ©Chris Wilkins/Texas BBQ Posse) What to do with barbecue leftovers? Always an important question. The best advice the Posse ever received on that front came years ago during a visit to Snow’s BBQ in Lexington, Tx. Reheat slowly in the oven with lots of butter, owner Kerry Bexley told us. Still solid advice. After a recent Saturday Posse feast at 2M Smokehouse in San Antonio, we found ourselves with lots of barbecue leftovers on a Sunday morning at our place in Austin. There…