1 c. packed dark brown sugar
4 oz. canola oil margarine (see Note)
1/2 stick (4 tablespoons) unsalted butter, at room temperature
1 large egg
1 t. vanilla extract
1/2 t. baking soda
1/2 t. salt
1 3/4 c. flour
2 t. root beer concentrate, such as McCormick's brand (see Note)
2 c. confectioners' sugar
2/3 stick (5 1/3 tablespoons) unsalted butter, at room temperature
2 to 4 T. water
1 t. root beer concentrate
For the cookies: In a large bowl, using a stand mixer or an electric hand mixer on medium speed, beat the brown sugar, margarine, butter and egg for several minutes until well blended and fluffy. Reduce the speed to low and add the vanilla extract, baking soda, salt, the flour in increments, and the root beer concentrate. The batter will be stiff. Cover and refrigerate for 1 hour.
Preheat the oven to 375 degrees. Line several large baking sheets with parchment paper (or just grease some). Drop the dough by teaspoonfuls, spaced about 2 inches apart (the cookies will spread), onto the prepared sheets. Bake for 6 to 8 minutes. Transfer to a wire rack to cool completely.
For the icing: In a medium bowl, combine the confectioners' sugar with the butter and mix well. In a measuring cup, combine the water and root beer concentrate; add to the sugar-butter mixture and mix well, adding more water as needed to reach the desired consistency. Spread on the cooled cookies. Let set for about 30 minutes before serving.
Notes: I have no idea what the canola oil margarine is, nor did I think these would be harmed if I omitted it. So instead of the 3/4 cups of fats it told me, I used one whole stick of butter. In addition, I could not find the concentrate at home, but found some "Root Beer Flavor Oil" at a Bosch store in Utah and used that. If you do that, I suggest using 3/4 t. of that in place of the 2 t. of concentrate in the cookies because it's stronger. I didn't ice the cookies, but a good way to eat these would be as a sandwich with vanilla ice cream in between (root beer float cookie anyone?). Trust me. It should also be noted that the recipe I was following said it would yield 24 cookies; I got 38.