Preheat oven to 350°F. Line 2 cookie sheets with silicone baking mats or parchment paper.
Cream butter and sugar for about a minute, or until fluffy, using a hand or a stand mixer.
Add egg, egg yolk, vanilla, baking soda, cream of tartar and salt and mix until combined, then mix in cocoa.
Add flour and mix until smooth.
Scoop 2 tablespoon size balls of cookie dough and place on cookie sheet 2-inches apart. If desired, roll them in granulated sugar beforehand.
Bake 9-14 minutes, or until no longer glossy. If you’ve rolled them in sugar, those will be slightly cracked. Do not over bake. Cool before frosting.
Make frosting: beat butter with cocoa and powdered sugar until crumbly. Add salt, vanilla and 1 tablespoon milk; mix until smooth, adding more milk as needed for spreadable consistency.
Frosting cookies as desired. Top with sprinkles if you want.