I discovered this combination years ago, and it is a favorite in place of regular mashed potatoes. The sweet and savory mixture plus the creaminess of the Yukon gold potatoes means it is naturally packed with flavor so you don't have to add a lot to them.
1 pound Yukon Gold Potatoes peeled
1 pound Sweet Potatoes peeled
1 tbsp Sea Salt
1/2-3/4 cup Milk - you can use any type of milk here, but I would avoid anything with a strong flavor. If you want to indulge, you can also use half & half
2 tbsp Butter
Salt & Pepper to taste
Cut the potatoes into equal size chunks. Put into a pot and cover with water and add the sea salt.
Bring to a boil and cook the potatoes until tender. Strain in a colander, reserving some of the starchy water.
Return the potatoes to the warm pot and begin to mash. Add the milk and butter. If it is still a bit stiff, add butter, milk, or some of the reserved cooking water. Mash until the consistency you like. Season with salt and pepper to taste.
Transfer to a casserole dish and serve. You can also refrigerate them at this point, and then when you want to serve them, reheat them in a 350 degree Fahrenheit oven for about 20 minutes.
If you don't want to use dairy, you can use chicken or vegetable stock plus olive oil instead of regular milk and butter.