In a small bowl, dissolve baking soda in room-temperature buttermilk. In a saucepan, melt butter and cocoa together at a low temperature. When smooth, add water. In a separate bowl, sift together sugar, flour, and salt. Add cocoa mixture and egg and mix at low speed. Add buttermilk mixture and vanilla and beat until smooth. Bake at 350 for 20 minutes or until inserted toothpick comes out clean
In a saucepan, melt butter at low temperature. In a separate bowl, mix together cocoa and sugar. Add melted butter, vanilla, and milk and beat at low speed until mixed. Frost cupcakes generously.