Heat milk and butter until butter is melted and milk is hot. NOT SCALDED! Pour into cake mixture.
Add 4 eggs and vanilla. Beat until batter is smooth. Pour into 2 8-inch cake pans. Bake following package directions. When done, remove from oven and let cakes cool on wire wrack.
For Lemon Filling: Mix together the 3 egg yolks and sweetened condensed milk until well blended. Add lemon juice, mix well. Refrigerate until set and cakes are completely cooled. Run a bead of frosting around top of bottom layer cake to hold lemon filling. Fill center of cake with the filling. Add top layer carefully, try to avoid "mashing" top, you don't want the filling to ooze out from between the layers. Frost sides and top of cake with remaining frosting. Refrigerate leftovers.