This was the star Xmas gift of a friend of my mother. She only parted with the recipe after tasting and wanting my star recipe :-)
When I made it the flour amount did not seem correct. She said 2 1/4 to 2 1/2 c. but had typed 1 1/4.
See my note below. Hope this is what you were looking for.

Chocolate Mint Bars

Base
2/3 c. margarine
4 sqs. unsweetened choc.
2 c. brown sugar
3 eggs, beaten
1 tsp. vanilla

Melt margarine and choc. in double boiler or carefully in microwave.
Mix in br. sugar, eggs & vanilla. Blend in by hand the following flour mixture:

1 1/4 c. flour ***
1 tsp. baking powder
1 tsp. salt

Spread evenly in greased 9 X 13 pan. Bake 350 for 25 min. Cool thoroughly.

Frosting
1/4 c. soft butter
2 c. powdered sugar
2 Tbsp. milk
1 tsp. peppermint extract
green food coloring

Beat all frosting ingredients together until smooth. Spread on cooled choc. base. Refrigerate while preparing glaze.

Chocolate Glaze
2 Tbsp. butter
2 sqs. unsweetened choc.

Melt together and dribble over cool, firm frosting. Spread evenly. (I found a feather worked best)

*** this is not enough flour. 2 c. is too much. 1 1/2 c. is a better amount.