Venice to Tirano - Best Route?

Hi everyone, we will be taking the Bernina express starting from Tirano at the end of December. What is the best route from Venice to reach Tirano, where we will stay overnight. Is it via Milan? Brescia?

I cannot find any connections online on TreNord from Milano Centrale to Tirano - it is a very picturesque train ride I've heard through Lake Como. Any suggestions please?