Stock cubes / bouillon cubes - as pictured in post. Chicken for base flavour, beef for flavour boost + makes gravy nice brown colour (if only chicken, gravy is pale).
Substitute with stock powder / granulated bouillon, but note that some brands of beef powder will not yield quite the same brown colour, will be slightly lighter.
If you only use chicken cube or powder, the gravy flavour will lose a touch of seasoning and will be a pale brown colour. Can make up for this with 1 tsp Dark soy sauce - it will stain the gravy and add a touch of extra flavour (this is a secret cooking trick for gravy!)
Gravy consistency - don't let it get too thick, it thickens more when sitting around to be served. If it gets too thick, just add a touch of water. Too thin, keep it on the stove, it should thicken quickly.
Make ahead and leftovers - transfer to container and place lid on, let cool then refrigerate for up to 5 days. Can also freeze - once thawed, reheat and give it a good mix. You'll need to loosen with water.