Preheat oven to 325F
Cream together butter and sugar until fluffy (about 5-7 minutes)
Beat in egg yolks one at a time
Alternate adding flour and condensed milk, scraping down the sides occasionally
In a separate bowl, whip egg whites until fluffy
Gently fold egg whites into batter until completely incorporated
Pour batter into a prepared pan and bake at 325F for 90 minutes
