Preheat oven to 350°F (175°C) and line two cookie sheets with parchment paper, or grease well.
In the large bowl of a standing mixer, beat together butter and sugars until light and fluffy, about 3 minutes. Beat in egg, then carrots.
In a separate bowl, whisk together oats, flour, cinnamon, salt, and baking powder. With the mixer speed on low, gradually add dry ingredients to carrot mixture. Beat until combined. Stir in raisins and walnuts.
Drop by teaspoonfuls onto baking sheets. Bake for 7-9 minutes, until set but still slightly under-baked. Allow to cool on baking sheet for 2 minutes, then transfer to wire racks to cool completely.
Make Icing: In the bowl of a standing mixer, beat all ingredients together until light and fluffy. Refrigerate until ready to use. When cookies are completely cool, spread about two teaspoons of icing on one cookie, and top with another to make cookie sandwiches. Store iced cookies in the fridge.
Per Carrot Cake Cookies
PER SERVING: about 215 calories, 3 g protein, 11 g total fat (6 g sat.fat), 26 g carbohydrate, 1 g fibre, 33 mg cholesterol, 152 mg sodium.