Whisk or sift together the flour, baking powder, and salt in a medium sized bowl. In a stand mixer, cream the butter, then gradually add the sugar, 1/3 cup at a time, then add the vanilla. Blend the eggs into the butter mixture, one at a time. Add the dry ingredients slowly, alternating with the milk, until fully mixed. Pour the batter into greased and papered cake tin(s). Bake at 350ºF/177ºC until toothpick comes out clean (30-35min for 6" cake tins). Cool fully.
Lemon Curd
Add all the ingredients except the butter in a saucepan and blend together. Place over low heat and cook, stirring continuously, until thickened, remove from heat. Add the butter slowly, stirring in completely. Cool.
Frosting
Place the egg whites and sugar in a stand mixer metal bowl, and place over a pot of simmering water. Cook until the temperature reaches 71ºC (160ºF). Remove from the heat, place on the stand mixer with a whisk attachment and whip until stiff peaks form. Beat in the salt, then add the butter 1 tablespoon at a time. Fold in the bumbleberry purée.