Whatever the box says .. or if you make your own .. whatever the recipe says... here is one I use
3 c. all-purpose flour
1 1/2 tsp. cream of tartar
3/4 tsp. each baking soda and salt
1 1/2 c. granulated sugar
3/4 c. butter at room temperature
1 c. milk at room temperature
3 lg. eggs
1 tbsp. vanilla
Combine dry ingredients in a large mixing bowl; blend well with wooden spoon. Add butter, milk, eggs, and vanilla. Stir with wooden spoon until entire mixture is moist.
Beat with electric mixer 4 minutes or until very well blended.
Pour into 2 lightly greased and floured 9-inch round cake pans. Bake in a preheated 375 degree oven 20-25 minutes until cake begins to pull away from sides of pan and pick inserted near center comes out clean.
Be careful not to overbake it or it will be tough and dry.