Preheat oven to 230 degrees C. Prepare baking trays by either greasing or lining with baking paper.
Cream together sugar, butter and eggs. Sift together flour, baking soda, baking powder, salt and cocoa. Add alternately the flour mixture to the creamed mixture, with the milk, boiling water and vanilla.
Drop teaspoon size pieces of dough onto trays. Bake 5-7 minutes. Allow to cool.
Combine flour and milk in saucepan until thick. Let cool. Cream butter, sugar, salt and vanilla. Add flour mixture. Beat until creamy.
Spread the base of one cake with filling. Sandwich together with a second cake on top.