I use a heaping spoonful (teaspoon from the cutlery set) of cocoa - as much as will balance on it - & a rounded spoonful of sugar. Mix those in a mug. Add just enough milk to make a thick slurry & mix until it is free of lumps. The small amount of liquid makes this easy. Then fill the mug with milk & microwave until hot enough. It's better to check it a few times & stir than to let it boil. For a treat I add a little 1/2 & 1/2.If I'm really wanting a chocolate fix, I'll stir in a few chocolate chips or a piece of bittersweet chocolate when it's hot. This is flexible & foolproof.
Hot cocoa and hot chocolate are different drinks. The very best advice I can give you: your liquid [milk, cream, water, almond milk, coconut milk etc] should be warm or even hot before whisking in cocoa (and sugar if you like). *Cocoa disseminates its flavor only when warm.* You should whisk cocoa for at least 10 minutes before drinking so as to "cook" out the "raw/floury" taste of cocoa. In winter I like to make a big batch and heat up individual mugs "to order."