|First version two months ago.|
My guess is that the silpat is serving as an insulator, causing the macarons to heat more slowly. Trying increasing your heat by 25 degrees or so and see what happens.
But if it's any comfort, I feel your pain. My last batch of macarons was a flop. I got too darn cocky and blew it. That'll learn me!
Soldier on, baker!