Beat the butter and sugar with an electric mixer until pale and fluffy. Add the vanilla extract and eggs one a time, mixing with each addition.
In a separate bowl, combine the flour, baking soda and salt. Add this to the batter in 2 additions, alternating with the buttermilk.
Dye the batter blue. Pour the batter into 2 greased and floured 9-inch round cake pans. Bake at 350F for 30 minutes, or until a skewer inserted into the cakes comes out clean. Cool in the pans for 10 minutes, then transfer to a wire rack and cool completely.