Preheat oven to 350 degrees F. Grease and flour 3 (9-inch) cake pans.
To make the cake: Using an electric mixer, cream butter until fluffy. Add sugar and continue to cream well for 6 to 8 minutes. Add eggs 1 at a time, beating well after each addition. Add flour and milk alternately to creamed mixture, beginning and ending with flour. Add vanilla and continue to beat until just mixed. Divide batter equally among prepared pans. Level batter in each pan by holding pan 3 or 4-inches above counter, then dropping flat onto counter. Do this several times to release air bubbles and assure you of a more level cake. Bake for 25 to 30 minutes or until golden brown. Cool in pans 5 to 10 minutes. Invert cakes onto cooling racks. Cool completely.
To make the frosting: Place sugar, cream of tartar or corn syrup, salt, water, and egg whites in the top of a double boiler. Beat with a handheld electric mixer for 1 minute. Place pan over boiling water, being sure that boiling water does not touch the bottom of the top pan. Beat constantly on high speed with electric mixer for 7 minutes. Beat in vanilla.
To make the filling: Place the ingredients in the double boiler over boiling water. Don't let top pan touch the water. Cook and stir until mixture begins to gel or thicken. Remove from heat and allow to cool.
To assemble, add 1 tablespoon of filling to the cake pedestal. Run hands along the side of the cake to remove excess crumbs. Place the cake layers on the pedestal, spreading filling between the layers and on top. Spread the sides and top of the cake with the remaining filling. Frost top and sides of cake with frosting. Garnish with a lemon wedge and a sprig of mint.