Don't overreact and go for the cheater brakes and such. Once you gain more experience and confidence in your bike handling skills, you will learn to love the downhills. The Salsa Casseroll is not a twitchy bike by nature, so what you are experiencing is fear causing you to give the bike the wrong inputs.
Fear is the mind killer.
Is your Casseroll the older model with road caliper type brakes (like mine) or the newer model with cantilever brakes? Either will work just as well, they just need to be set up correctly. You may want to practice on some gentle descents to get a better feel for how to control the bike while descending.