Cut sweet potatoes into smaller pieces so they cook faster. Add to a large pot and boil until they're soft - about 15-20 minutes. Once a knife easily pierces through, they're done.

About 10 minutes into boiling the sweet potatoes, add the butter into a small skillet or saucepan. Cook over medium heat until the butter is golden brown and smells nutty. There's a fine line between browned butter and burned butter, so watch it closely. You don't want it to go too dark. Add the chopped sage when you think the butter is about done. Add the garlic in after you've cooked the sage for about a minute. Let the garlic cook for about 15 seconds, then remove the brown butter from the heat.

Drain the sweet potatoes and mash them with the brown butter, cream, garlic, and salt and pepper. Serve immediately.