Apple Blondies with Brown Sugar Frosting

Ok, this might be my favorite recipe ever! Everyone looooves these. I even had a friend say it was the 'best dessert he's ever had'. Sorry, no photo- but we ate them too quickly!
Here is where I found the recipe: http://kayteskitchen.blogspot.com/2007/10/apple-blondies-with-brown-sugar.html
I've made a few adjustments to the original recipe just because this wasn't my first time preparing it and I thought the original amount of frosting was a bit much.

Apple Blondies with Brown Sugar Frosting
Blondies:
2/3 c. butter softened (I used 1/3 c. margarine, and 1/3 c. butter)
2 c. brown sugar
2 eggs
1 t. vanilla
2 c. flour
2 t. baking powder
1/4 t. salt
1 cup chopped apples (I used big chunks)
1 1/2 t. cinnamon
*3/4 c. chopped walnuts (I did not use nuts)

Brown Sugar Frosting:
*The original recipe calls for 2x the amount of frosting and for it to be spread over the bars. I have halved the recipe and drizzled the frosting over the top (looks prettier) and even so had a lot of frosting left over.

1/4 c. butter or margarine
1/2 c. brown sugar
1/8 c. skim milk
1 c. confectioner's sugar

Blondies:
In a large bowl, using and electric mixer (I did this by hand) on medium speed, beat butter, sugar, eggs and vanilla until thick and smooth, about 3 minutes. Combine flour, baking powder and salt. Add to the butter mixture on low speed, mixing until blended. Stir in apples, cinnamon and nuts, mixing well. Spread evenly over greased 9x13 pan.
Bake in Preheated oven at 350 degrees until set and golden, 25-30 minutes. Cool completely on rack.

Frosting:
In small saucepan over low heat, melt butter. Stir in brown sugar and milk. Bring mixture to a boil then remove from heat and cool to lukewarm. Stir in confectioner's sugar, mixing until smooth. Drizzle over bars. Let stand until frosting is firm enough to cut into bars or squares.