General aviation pilot reported low altitude alert from ATC during initial approach. Pilot corrected altitude; then continued approach.

Narrative

I was coming into BTV for the first time (never been to this airport). Approach told me that they would vector me for an approach to runway 15 where I could have used an ILS; but conditions were VFR and the winds favored runway 19 so I requested a visual approach into runway 19 (also; my heading toward the airport was almost 190 degrees). ATC vectored me toward runway 19 for a straight in visual approach at 2250' MSL. I was cleared for the visual approach and told to contact the tower. I had the airport in sight and began descending. I had looked at the airport elevation earlier (335'); but I had it in my mind that it was about 100' so I set my autopilot to descend to 1200' MSL. I was still about 4 miles away from the runway when tower told me to 'check my altitude immediately'. I had just leveled off at 1200' and I thought the tower controller was concerned that I would descend further. I held the 1200' for a few moments until it was clear that I should descend to the runway (which came very quickly in the turboprop). As soon as I landed I looked at the airport elevation and thought about what just happened. I immediately realized my error and that I was less than 1000' AGL. Later; I uploaded this flight to an app and looked at this flight even closer. This was a big mistake and I'm surprised that I did this. I should have stayed closer to 1500' AGL (about 1800' MSL) until I was closer to the airport. I honestly don't know why I did this but I can say for certain; that I will be MUCH more conscientious about this in the future.