First 8 ingredients are for cake. In large mixing bowl, cream butter and sugar until light and fluffy. Add the egg and egg yolks; mix well. Beat in vanilla. Combine the flour and baking powder; add to the creamed mixture alternately with milk, beating well after each addition.
Pour into a greased and floured 9-in. round baking pan. Bake at 350-deg. for 25-30 minutes or until a toothpick inserted near the center comes out clean. Cool in pan for 10 minutes before removing to a wire rack to cool completely.
For filling, combine sugar and flour in a heavy saucepan. Stir in water until smooth. Cook and stir over medium-high heat until thickened and bubbly. Remove from heat. Stir in a small amount of hot filling into egg yolks; return all to the pan, stirring constantly. Bring to a gentle boil, cook and stir 2 minutes longer. Remove from the heat. Gently stir in lemon juice, butter and lemon peel. Cool to room temperature without stirring. Place cake on a baking sheet; spook filling on top of cake to within 1/2-inch of edge.
For meringue, let egg whites stand at room temperature for 30 minutes. In a small mixing bowl and with clean beaters, beat egg whites and cream of tartar on medium speed until soft peaks form. Gradually add sugar, 1 tablespoon at a time, beating on high until stiff peaks form. Carefully spread over filling, sealing to edges of cake. I begin at the outer edges, fill to middle. Bake at 350-deg. for 12-15 minutes or until lightly browned.