Preheat your oven to 350°F (175°C). In a large bowl, take the softened butter and cream it together with the brown sugar until it’s light and fluffy. Add the eggs and vanilla to the mixture, stirring well.
In another bowl, whisk together the flour, oatmeal, salt, cinnamon, and baking soda to combine them nicely. Gradually add this dry mixture to the wet mixture, stirring until just combined. Gently fold in the finely diced rhubarb to add that delightful tang.
Scoop tablespoon-sized amounts of the dough onto a cookie sheet, leaving some space between each dollop. Bake in your preheated oven for about 15-18 minutes, or until they are lightly browned. When they’re done, you can sprinkle them with coarse sugar if you’d like that extra sparkle. Let them cool for about 2 minutes on the tray before transferring them to a wire rack to cool completely.
For the cream cheese frosting, beat together the room-temperature cream cheese and four tablespoons of softened butter until it’s creamy and fluffy. Gradually add in the powdered sugar and the vanilla, continuing to beat until smooth. If necessary, add milk a little at a time until you reach the perfect spreadable consistency.