Preheat oven to 350F and line 2 baking sheets with parchment paper or silicone baking mats.
Combine the flour, baking powder, baking soda, and salt in a bowl then whisk together and set aside.
Cream the butter using an electric mixer then add the sugar and mix on high until light and fluffy.
Add the egg and vanilla then mix once more until smooth. Scrape the bowl down and mix once more then add the sour cream and mix until fully incorporated.
Add the dry mixture to the butter mixture and mix on low until just combined.
Scoop heaping tablespoon-sized portions of dough out and transfer to your prepared cookie sheet leaving about two inches of space between. For a more even bake, dampen your hand and pat the cookies into shape, pressing each one down slightly.
Bake at 350F for about 11 minutes. The bottom of the edge will Just be turning golden when they are done.
For the buttercream
While the cookies bake cream the butter and mix in two cups of powdered sugar.
Add the vanilla and two tablespoons of milk. Mix until smooth. Add more milk if needed to thin the frosting out into an easily spreadable consistency. You may dye the frosting with food coloring if desired.
Use a spatula to spread the buttercream onto the cooled cookies and enjoy!
Make sure your butter is softened (at room temperature).
All other ingredients (sour cream and egg) should be at room temperature for best results. Just remember to remove them from the fridge around 1 hour in advance.
For even-sized cookies use a 2 tablespoon cookie scoop.
I like to use damp hands to pat the cookies down slightly and give them an even shape after scooping them onto a cookie sheet, this just helps to make them more uniform.
Make sure to line your cookie sheet with baking parchment or a silicone mat.
I recommend using gel food coloring if you'd like to create different buttercream colors.
If you want to add different flavors you can try adding lemon or orange zest to the cookie dough and add a little fresh citrus juice to the buttercream frosting. Also, check out my Chocolate Buttercream Frosting, and Sugar Cookie Icing for other frosting ideas.
Make sure your cookies are completely cooled before frosting or the frosting will melt off.
Storage - cookies will keep well in an airtight container for up to 3 days or can be frozen for up to 3 months.