Before I start mixing, I like to measure everything out first so I am not scrambling once the batter is wet. Spoon the flour into your measuring cup rather than scooping it directly from the bag, since scooping packs in too much and can make the cake dense. Having everything ready also means you can move from dry to wet ingredients without any pauses.
Add the flour, sugar, cocoa powder, baking soda, and salt directly into your mug. Use a fork or small whisk to stir everything together until you no longer see streaks of cocoa powder. Taking an extra few seconds here helps avoid any dry pockets hiding at the bottom of the mug later.
Pour in the milk, canola oil, water, and vanilla extract right on top of the dry mixture. Stir well, making sure to scrape around the bottom and sides of the mug so nothing is left unmixed. The batter should look smooth and glossy, with no lumps of flour remaining.
If you're using chocolate chips, sprinkle them on top now instead of stirring them in, so they don't sink and burn. Microwave on high for about 60 to 90 seconds, checking at the one-minute mark. The cake is done when the top looks set and a toothpick comes out mostly clean.
Let the mug sit for two to three minutes before digging in, since the inside stays extremely hot right out of the microwave. This short rest also lets the texture settle into that soft, cake-like crumb instead of feeling gummy. Once it has cooled slightly, grab a spoon and enjoy it warm.