When I was a kid, I'd fill the glass halfway with Quik powder, fill the rest with milk, and then stir it near the top enough so only about a third to half of the powder dissolved. I'd drink the chocolate milk, then eat the remaining milk-soaked chocolate powder paste with the spoon. I'd also do this with malted milk, sometimes together.
Hershey's syrup was good for a quick dose, just tilt your head back and squirt a shot right into your mouth like a water bottle.
Nowadays I just boil some water, sugar, cocoa powder, and vanilla extract to make my own chocolate syrup. Can make it as dark and thick as I like that way.