ZBW Center Controller reported they transposed digits in an approach clearance and descended an aircraft to an altitude below the Minimum Vectoring Altitude.

Narrative

I cleared Aircraft X for the RNAV 18 approach into BML. As I was issuing the approach clearance; I noticed a SPCI WX observation at the field; so I combined his approach clearance along with reading him the WX at the field. Instead of giving him a crossing restriction of 063 over ELSEQ; I said cross ELSEQ at 036 cleared approach. The pilot (after reviewing the falcon and voice) read back the 036 over ELSEQ. I did not catch only my transposing of the altitudes on the initial Clearance; nor did I catch the readback. About 2 min later I saw the aircraft descending through 058 and corrected the issue. I climbed him back up to 063 and reissued the correct restriction over ELSEQ. At the time; the sector was very busy. I had asked for a D-side several times; unfortunately we did not have someone to come back immediately. There was not only a lot of traffic volume but also a lot of phone calls from FD (Flight Data) for clearance requests. I thought I had said 063; and I thought I heard 063. There was an expectation bias on my part about what I wanted to happen and what happened in reality. Being busy probably did not help the situation however in the end I just made a mistake.Recommendation: The only thing that could have prevented this; as it was human error; would have been for my D-side to have caught the read back or myself at the time of the initial clearance and corrected it right away.