1. Preheat oven to 350 degrees.
2. Grease 2 (8") Cake Pans with butter and line bottoms with parchment paper, butter parchment paper.
3. Cream cold butter and add sugar. Beat until light and fluffy on medium speed.
4. Scrape sides of bowl and add egg yolk and eggs one at a time beating well between each addition. Scrape sides of bowl as needed.
5. Add vanilla, baking powder, baking soda and salt.
6. Mix until well combined. Scrape sides of bowl.
7. Add flour, yogurt and milk.
8. Mix on medium low speed just until combined being careful not to over mix.
9. Divide batter evenly between cake pans and bake for 26-30 minutes or until a toothpick inserted into the center of each cake comes out clean.
10. Remove cake from oven and cool for 10 minutes in pans. Then invert onto a cooling rack and finish cooling to room temperature.
11. To make Mocha Buttercream Frosting: Cream together butter, cocoa powder and salt until smooth.
12. Add coffee, half & half or milk, vanilla extract and powdered sugar.
13. Mix on medium low speed until well combined and smooth. Scrape sides of bowl as needed.
14. For a level cake trim top off of cake.
15. Spread 1/3 of the Mocha Buttercream Frosting. Top with second layer and frost sides and top of cake.
16. For the cleanest cut refrigerate cake for 1-2 hours to help frosting set before serving.