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Heat oven to 180C/160C fan/gas 4 and line baking trays with parchment. Mix flour, cocoa, bicarb, sugar and a pinch of salt. Whisk the egg, oil and buttermilk together, then mix into dry ingredients with 75ml boiling water. Spoon 20 spoonfuls onto trays, well spaced. Bake for 12 mins or until firm to touch, peel off trays. Cool.
For the glaze, sift icing sugar and cocoa into a small pan. Gradually stir in cream followed by 100ml boiling water. Heat gently until syrupy.
Sandwich 2 cakes together with the frosting, spoon over some glaze and decorate with sprinkles.