Mix flour, sugar, cocoa and salt. Add eggs, oil, vanilla and buttermilk. Beat.
Dissolve baking soda in hot water. Stir into batter. Spoon into prepared pans. Bake for 15-20 minutes. Cool before filling.
For filling, cream butter and sugar. Stir in vanilla. Add cream gradually until filling is of desired consistency. Use decorator bag and tip. Fill bag with filling. Insert tip into cupcake and squeeze to fill.