preheat the oven to 350 degrees f and line a baking sheet with parchment paper or silicone.
in a medium bowl, whisk flour, bran, baking soda, salt, and spices.
in a large bowl, use a stand mixer, hand mixer or arm muscle to combine butter and sugars mix until smooth and fluffy. add egg and vanilla extract and mix until combined.
next, mix in the shredded carrots.
slowly add the flour mixture until mostly combined. then stir in oats until completely combined.
refrigerate, covered, at least 1 hour.
using a small cookie scoop or eyeballin' it, drop cookie dough in balls no larger than a quarter, 2 inches apart, onto baking sheet. you can flatten the tops a little if that interests you. bake for 8-10 minutes or until middles set and edges slightly colored. cool completely on wire racks.
while cooling, mix all frosting ingredients at room temperature together with a hand or stand mixer until completely combined. if the frosting isn't firm (aka standing in peaks) add a bit more powdered sugar. fill piping bag with a medium size round tip (think a pencil eraser size).
once the cookies are cool, pipe as much frosting as you can possibly handle on the flat side of half of your cookies, then sandwich it off!
these keep wonderfully refrigerated for several days, but i guarantee you won't have them that long!