Place sweet potatoes in a large saucepan, cover with water to 2 inches above potatoes
Bring potatoes to a boil over high heat
Once at a boil, reduce heat to medium-low and simmer until tender, about 18 minutes.
Remove from heat. Drain potatoes and return to saucepan.
Combine butter and olive oil in a small skillet and over medium heat
Cook until browned and fragrant, about 3 minutes. Remove from heat, stir in thyme.
Add milk, salt and pepper to taste to sweet potatoes and whisky to combine
Mash everything together with a potato masher to desired consistency.