Cake: In a small bowl, combine cocoa and hot water, cool, in a large bowl, cream butter and brown sugar until fluffy, add eggs one at a time, beat well, combine flour, baking soda, baking powder and salt; gradually add to creamed mixture alternately with buttermilk and cocoa mixture, beating well after each addition, stir in vanilla, beat.
Pour into three greased and floured and wax paper lined 9 inch round baking pans. 350 20 to 25 minutes, test. Cool at least 10 minutes, turn out on wire rack, cool before icing.
Filling: In 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 heat and stir in vanilla. Cool completely. In small bowl, cream butter, gradually beat in cooled mixture. Place one cake layer on serving plate, spread with half of the filling and repeat layers.
Glaze: In a small saucepan, melt butter; stir in cocoa until smooth, add milk bring to a boil, remove from heat. With an elec. mixer, beat in confectioners' sugar until blended. Cool for 15 minutes or so; spread over top of cake. Garnish with choc. curls if desired. Refrigerate until serving.