Preheat oven to 180 C / Gas 4. Grease two 23cm sandwich tins.
Sift flour, baking powder, salt, bicarb and 300g caster sugar together in a large mixing bowl. Add 100g butter and 300ml evaporated milk. Beat at medium speed with an electric mixer for 2 minutes. Beat in the eggs and beat for 2 minutes longer. Spread the mixture evenly into the prepared tins. Drizzle 60g of the melted chocolate in a spiral on top of each cake. Feather lines with a knife to form a web pattern.
Bake for 30 to 40 minutes or until a skewer inserted in the centre comes out clean. Let cakes cool in tins for 10 minutes then remove from tins and let cakes cool completely.
To make filling: Combine the 60g melted chocolate, 275g butter, 275g caster sugar, 175ml evaporated milk and the vanilla together and beat with an electric mixer until smooth.
To assemble cake: Cut each cooled cake layer in half horizontally. Spread 1/4 of the filling between each layer making a 4 layer cake with a web design on top. Ice cake with the remaining filling.