Heat up a deep skillet (that has a lid), over medium heat. Place your butter into the skillet and allow it to melt. Now sprinkle in your brown sugar and white sugar. 

Use a wooden spoon to mix the sugars into the butter until they are fully incorporated. Now add in your vanilla extract, lemon juice, cinnamon, ground nutmeg, salt, and allspice. Mix this up thoroughly and allow the mixture to simmer for at least one minute. 

Gently add your thinly sliced sweet potatoes into the candied sugar mixture. Use your spoon to gently stir the sweet potatoes around so that they are completely coated with the candied sauce.

Try to spread them out so they are in one even layer, and cover your skillet with a lid. Bring the candied yams to a light boil for about 5-7 minutes, remove the lid and stir them around. 

Make sure they are spread out in one even layer again, and place the lid back on the skillet. Turn the heat down to medium-low, and simmer the potatoes for another 20 minutes, or until they are fork tender. 

Remove the lid for the last 5 minutes of cooking to allow the sauce to thicken up even more. 

Serve right away, and Enjoy!