Preheat oven to 350 degrees F. Grease and flour two 9x2 inch round cake pans.
Cream the butter in bowl of a stand mixer until fitted with a paddle attachment. Add sugar and continue beating until light, scraping bowl often. Add the salt and continue creaming for another 5 minutes at medium high speed.
While the butter and sugar are creaming in the stand mixer, sift flour and baking powder together. Remember, the flour should have already been sifted before measuring. Set aside in a bowl.
Combine the eggs and yolks together in a separate bowl.
Combine the milk and vanilla in a measuring cup.
When the creamed mixture start to look fluffy, turn mixer up and add the egg/yolk mixture gradually by spoonfuls (I did 6-8 additions), making sure mixture comes together fully after each addition. Scrape down bowl and continue mixing until uniform.
By hand or with very lowest speed of blender (I always do this by hand, especially when using all purpose flour), add the flour mixture and milk mixture alternately, beginning with flour and ending with flour so that you add dry, wet, dry, wet, dry. When all flour and liquid is incorporated and batter appears uniform, divide batter among the pans.
Bake at 350 degrees F until done. My cake was done in about 28 minutes, but all ovens are different so check at 18 min.Cake is done when middle bounces back and/or small sharp knife inserted in center comes out clean and sides pull away from pan.
Frosting: Off the heat, combine the sugar, flour and cocoa in a small (2 qt) saucepan. Gradually add milk and whisk until smooth.
Put the mixture over medium heat and cook, whisking constantly, until mixture thickens and boils. Reduce heat to low, cook 2 more minutes, whisking all the while. Remove saucepan from heat; cool completely. This should take 40 minutes to an hour. At this point, you might want to go ahead and melt your chocolate so it too will be cool.
In large bowl with mixer at medium speed, beat softened butter until creamy. Gradually beat in cooled milk mixture, vanilla and melted chocolate. Depending on the temperature of your ingredients, you’ll either have a beautiful frosting or a big goopy mess. Never fear! If what you’ve got is too goopy (this is usually due to ingredients being too warm), stick the whole mixing bowl in the refrigerator for another 40 minutes or until the mixture is thoroughly chilled. Remove it when chilled, put it on the stand mixer with the whisk attached, and beat until it lightens in color and fluffs up. Frost the cake.