Dissolving cocoa powder in milk
Aw, you youngsters, spoiled with your Nestle Quick... :)

To mix cocoa powder with a liquid (or really, to mix any powder with a liquid - salt and granulated sugar aren't powders), you need to make a slurry by mixing a small part of the liquid into all of the powder. Then you can dilute the slurry with the rest of the liquid.

Note that if you're using sugar for sweetening, it helps to add it to the cocoa powder before you add any liquid, because the sugar helps the dissolving. However, since stevia is so much sweeter than sugar, you use too little of it to make any difference to the dissolving process. Thus, you can go ahead and add it afterward, so you can adjust the sweetness better.