Start by heating your oven to 350°F so it's fully ready when your batter is. While it warms up, spray the inside of four 6-ounce ramekins with cooking spray so nothing sticks later. Set them on a rimmed baking sheet, which makes transporting them in and out of the oven much easier.
Prepare the brownie mix following the instructions on the box, using the vegetable oil and eggs. I just use one bowl and a spatula, no mixer needed, and it comes together in a couple of minutes. Give it a good stir so everything is fully combined and smooth.
Spoon about ½ cup of batter into each ramekin, then press two chocolate truffles side by side into the center of each one. Top that with another ¼ cup of batter, smoothing it over with the back of a spoon so the truffles are fully covered. This layering is what creates that molten pocket once it bakes.
Slide the baking sheet into the oven and bake for about 25 minutes, until the tops look set and dry. The center will still be soft and gooey underneath, which is exactly what you want. Every oven runs a little differently, so keep an eye on the last few minutes.
Once out of the oven, let the ramekins rest on a wire rack for about 5 minutes. This short cooling time keeps the molten center intact instead of collapsing when you dig in. It also makes them just cool enough to handle without burning your fingers.