In a small bowl, combine cocoa and hot water until smooth; cool. In a large mixing bowl, cream butter and brown sugar. Add eggs, one at a time, beating well after each. Combine the flour, baking soda, baking powder and salt; add to creamed mixture alternately with buttermilk and cocoa mixture. Stir in vanilla.
Pour into three greased and waxed paper lined 9 inch round baking pans. Bake at 350 for 20-25 minutes or until a toothpick comes out clean. Cool for 10 minutes; remove from pans to wire racks to cool completely.
In a small saucepan, combine sugar and cornstarch. Gradually stir in milk until smooth. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from the heat; stir in vanilla. Cool completely. In a small mixing bowl, cream butter. Gradually beat in cooled mixture.
Place one cake layer on a serving plate; spread with half of the filling. Repeat layers. Top with remaining cake.
For glaze, in a small saucepan, melt butter; stir in cocoa until smooth. Add milk. Bring to a boil. Remove from the heat. With an electric mixer, beat in confectioners' sugar until blended. Cool for 15-20 minutes; spread over top of cake. Garnish with chocolate curls if desired. Store in the refrigerator.