Well as the name days B-TECH means bachelor in technology. So their is no course like in which you can do mathematics only. Every stream have some technology involved with it and in those technology you can find some mathematics part like in mechanical, aeronotical, aerospace and all the branches have a little bit of mathematics involved in them. So their is no branch like in which you have mathematics only.

Yes of course. You can go with B.Tech in Mathematics and Computing Engineering. It is one of the heighest demanding and have great placements.It is available in many quality colleges. Here is a link of list of colleges that offer B.Tech(MCE) 

https://engineering.careers360.com/colleges/list-of-mathematics-and-computing-engineering-colleges-in-india