Instant Pot:

1. Pour 1 cup of water into the bottom of Instant Pot. Add potatoes into a steamer basket* and lower the steamer basket into the Instant Pot.
2. Cover Instant Pot and secure the lid. Make sure valve is set to sealing. Set the manual/pressure cook button to 10 minutes. When time is up let the pot sit for 5-10 minutes and then move valve to venting. Remove the lid.
3. Remove the steamer basket. Dump out liquid in bottom of Instant Pot. Pour the potatoes from the steamer basket into the bottom of Instant Pot liner. 
4. Add in the butter and let the heat from the potatoes melt it. Add in the salt and pepper. Mash the potatoes until creamy. Stir in the egg (the residual heat from potatoes will cook the egg). Stir in the milk. 
5. Salt and pepper to taste. Serve and enjoy. 

Slow Cooker:

1. Add water, potatoes, butter, salt and pepper into slow cooker. 
2. Cover and cook on high 4-5 hours or until potatoes are tender. Do not drain water.
3. Mash the potatoes and add in the egg yolk and enough milk to reach desired consistency.
4. Salt and pepper to taste. Serve and enjoy.