Mix together all ingredients until well combined. Scoop batter into muffin tin filled with muffin/cupcake liners.
Scoop batter until 2/3 to 3/4 full.
Place in oven and bake for 16-22 minutes. Insert a toothpick in the center to check if it is done.
Remove from oven and place on cooling rack until completely cool. Prepare glaze.
In a small bowl, beat together the butter and cream cheese with an electric mixer or hand beater. With the mixer on low speed, add the powdered sugar until smooth and creamy. Stir in lemon juice and lemon zest (if using).
Add 1/2 to 1 teaspoon of milk to get your desired consistency. I used 1/2 teaspoon.
Once the glaze is done and donuts are cool, dip the tops of the muffins into the glaze and turn over, back onto the cooling rack.