Preheat oven 180°. Lightly grease bottom and side of two 6 x 2-inch round cake pans. Line bottom of each pan with round of parchment paper. Grease paper and dust pans with flour, tapping out excess. Set aside.
Whisk together flour, baking powder and salt in a medium bowl. Combine milk, lemon zest and vanilla extract in a separate bowl. Beat butter and sugar in the bowl of an electric mixer on high speed about 3 minutes or until light and fluffy. Reduce speed to low and alternately add flour mixture and milk mixture in several additions, starting and ending with flour mixture just until combined. Set aside.
In a clean mixing bowl, beat egg whites until stiff peaks form. Gently fold beaten egg whites into cake batter in 3 additions until evenly combined. Evenly divide batter between cake pans and bake 30 to 35 minutes or until golden and cake tester inserted into centre of cake comes out clean. Cool in pans on wire rack 10 minutes. Invert to cool completely.
Frosting and Decoration
Beat butter and vanilla with an electric mixer on high speed until light and fluffy, about 3 minutes. Decrease speed to medium-low and add sugar, about tablespoon at a time, mixing thoroughly between additions. Add lemon curd and beat just until blended.
Place one cake layer on a cake plate or stand. Spread about 1/2 frosting on cake layer leaving 1/2-inch border. Top with single layer blackberries. Spread thin layer of frosting on other cake layer and place frosting side down on top of blackberries and press down gently to secure. Frost top and sides of cake with thin coat of frosting and refrigerate 15 minutes to set. Frost cake with remaining frosting. Top cake with remaining blackberries. If not serving cake immediately, refrigerate until ready. Remove from refrigerator 15 to 30 minutes before serving.