Beat the butter in the bowl of an electric mixer until light and creamy. Beat in the Natvia and eggs one at a time until combined. Sift over the flour and baking powder and whisk until you have a silky batter. Add 1 tbs milk at a time if the batter is too thick, until you get the silky consistency
Divide batter 2 x 20cm greased and lined cake tins and 1 10cm cake tin. Smooth the top with a spatula.
Bake for 20 mins, until a skewer inserted in the middles comes out clean, without any crumbs or gooey batter on it. Stand for 10 minutes in the tin before removing to a wire rack to cool completely.
To make the buttercream, beat the butter until light and creamy, add the powdered Natvia, and beat. Add the mashed blakckberries and beat until combined and fluffy.
Spread the cooled cakes with the buttercream and layer them up, filling the middle with fresh blackberries.