Add the oil, brown sugar, and salt to a 2.5- to 3-quart microwave-proof bowl with a vented lid. Microwave just until the oil is melted, 20-30 seconds. Stir to combine well, and then add the popcorn kernels. Stir to coat each kernel with the mixture, and then spread the kernels evenly over the bottom of the bowl. Place the lid on the bowl, and microwave on high for approximately 3 minutes. As soon as there are 1-2 seconds between pops, stop cooking. Microwave times will vary and the sugar will burn quickly at the end so err on the side of under-popping until you know how long this will take in your microwave. (I’ve done this so many times that I know 2:55 is perfect in my microwave.)