Pauline's Congo Bars

2/3 c. butter
1 c. brown sugar, packed
1 1/4 c. sugar
3 eggs at room temperature
2 3/4 c. flour
2 1/2 tsp. baking powder
1/2 tsp. salt
1/4 c. sour milk*
1 tsp. vanilla
1 lb. chocolate chips
1/2 c. chopped nuts (optional)

*sour milk, 1 T vinegar and add enough milk to make 1/4 c. of liquid. Cream butter, sugars and eggs (add eggs whole, one at a time), when well blended, light and fluffy, add sifted flour mixture and sour milk a little at a time until all is blended and smooth. Stir in chocolate chips and nuts. Pour into a greased and floured 9x13 pan (a jellyroll pan works best for this recipe). Bake at 350 for 30 minutes. Check with toothpick, it is ready when the toothpick comes out clean.