Preheat oven to 180 C / Gas 4. Grease and flour two 23cm (9 in) round cake tins; cover bottoms with baking parchment.
In a large bowl, combine flour, 400g sugar, cocoa, baking powder, bicarb and salt. Add eggs, milk, oil and 1 tablespoon vanilla; beat until well blended. Pour cake mixture into prepared tins.
Bake for 35 minutes, or until a skewer inserted into centre comes out clean. Cool layers in tins on wire racks for 10 minutes. Loosen edges, and remove to racks to cool completely.
To make the cherry filling: Drain cherries, reserving 125ml juice. Combine reserved juice, cherries, 200g sugar and cornflour in a 2 litre saucepan. Cook over low heat until thickened, stirring constantly. Stir in 1 teaspoon vanilla. Cool before using.
To make the icing: Combine whipping cream and icing sugar in a chilled bowl. Beat with an electric mixer at high speed until stiff peaks form.
With long serrated knife, split each cooled cake layer horizontally in half. Break one split layer apart into crumbs; set aside. Reserve 1/4 of the whipped cream icing for decorating cake; set aside. Gently brush loose crumbs off top and side of each cake layer with pasty brush or hands.
To assemble: Place one cake layer on cake plate. Spread with whipped cream icing, then top with 180ml cherry topping. Top with second cake layer; repeat layers of whipped cream icing and cherry topping. Top with third cake layer. Ice side of cake. Pat reserved crumbs onto icing on side of cake. Spoon reserved icing into pastry bag fitted with star tip. Pipe around top and bottom edges of cake. Spoon remaining cherry topping onto top of cake.