This recipe makes about 1/2 cup of chocolate sauce, but you can easily make up a big batch. I am keeping mine in a reused peanut butter jar. A clean squeeze bottle (ketchup, syrup, or even the Hershey's one you used the last of) would work great, but I didn't need that much sauce.
Homemade Chocolate Sauce Recipe
1/3 cup unsweetened cocoa powder
1/2 cup sugar
1/2 cup water
Mix the ingredients in a small saucepan. Cook over medium-low heat, stirring constantly. Bring to a boil and allow to boil 2-5 minutes, until the sauce thickens. If it starts to smell like cooked chocolate, take it off immediately. It will thicken more as it cools.
Keeps well in the refrigerator for a month. The solids might start to separate and settle after a while, but you can heat it up again to remix if you want.