Combine ½ cup sake; ¼ cup soy sauce; ¼ cup water; 3 tablespoons rice vinegar; 3 tablespoons sugar; ¼ ounce dried porcini, rinsed; 1 tablespoon instant dashi; 1 tablespoon oyster sauce; 1 thinly sliced small shallot; and 2 smashed and peeled garlic cloves in small saucepan and bring to boil over medium heat.
Lower heat slightly and cook at strong simmer for 5 minutes.
Remove from heat and let steep for 15 minutes.
Combine 1 tablespoon cornstarch and remaining 1 tablespoon rice vinegar in small bowl.
Strain sake mixture through fine mesh strainer into second small saucepan.
Whisk cornstarch mixture into sake mixture.
Bring to boil over high heat and cook until fully thickened, 30 to 60 seconds.
Let cool to room temperature, about 30 minutes.
Transfer to squeeze bottle or airtight container.
Store at room temperature for up to 3 weeks.
