Let butter, eggs and milk stand at room temperature for 30 minutes. Butter, line with wax paper, butter, and lightly flour five 9x1-1/2-inch round baking pans.
Preheat oven to 350 degrees F. In a large mixing bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Gradually add sugar, beating until well combined. Add eggs, one at a time, beating well after each addition. Beat in salt and vanilla. Combine flour and baking powder. Alternately add with milk to butter mixture. Spread batter in prepared pans, using about 1-1/3 cups batter per pan.
Bake about 15 minutes or until tops spring back when lightly touched. Cool in pans on wire rack for 5 minutes. Remove from pans; remove and discard waxed paper. Cool completely on wire racks. Frost with Chocolate Icing. Makes about 16 servings.
In a 4- to 5-quart Dutch oven stir together sugar, cocoa powder, and salt. Add butter, cut up, evaporated milk and whole milk. Cook and stir over medium heat until sugar is dissolved. Bring to a boil over medium-high heat, stirring occasionally. Boil gently for 4 minutes, stirring often. Clip candy thermometer to side of pan. Reduce heat and cook to 230 degrees F, stirring occasionally, about 15 to 20 minutes.
Remove from heat; stir in 2 tsp. pure vanilla. Cool slightly, without stirring, for 20 minutes or until thermometer registers between 165 degrees F and 170 degrees F and icing begins to thicken to the consistency of hot fudge sauce (pourable, but not runny). Generously ice between layers, and on top and sides of cake by pouring on some of the icing and spreading with an offset spatula. Makes 4-1/4 cups.
Homemade Baking Powder
In a small bowl combine cream of tartar and baking soda. Sift together 3 times. Transfer to a clean, tight-sealing jar. Store at room temperature, away from sunlight, for up to 6 weeks.