Cream butter,shortening and sugar well. Add one egg at a time, beating well. Sift flour,baking powder and salt. Add flour and milk alternately to the sugar mixture, Add flavoring,pour into greased large oblong pan.
Preheat oven 350 Bake 1 hour are until done. Cool cake in pan, cut into squares,layer in a large bowl, pour lemon filling on each layer. Cake may be done before 1 hour.
Directions for Lemon Filling: In a large sauce pan or double boiler , combine sugar, flour, salt, mix well. Beat egg yolks, milk, water together, then whisk into sugar mixture. Cook over medium heat stirring constantly, until mixture thickens. Remove from heat and stir in grated lemon rind, lemon juice and butter, cool then pour over cake. Sometime i don't use the grated lemon rinds.