For the cookies, beat together the butter, brown sugar, and regular sugar in a large bowl until light and fluffy. Beat in the egg and vanilla until well combined. Add in the flour, graham cracker crumbs, cinnamon, baking powder, baking soda, and kosher salt and stir just until combined. Transfer the dough onto a piece of plastic wrap, flatten into a one-inch thick dish, wrap it tightly, and place in the fridge for two hours or up to three days.
When ready to bake, preheat your oven to 350ºF and line a baking sheet with parchment paper. Lightly flour a work surface and roll out the chilled dough to about 1/8-inch thick. Using a small cookie cutter, a pizza wheel, or a knife, cut out small bite sized cookies (shapes, squares, or whatever you’d like) and transfer them to the prepared baking sheet, leaving about 1/2-inch of space between each cookie. Bake the cookies in the preheated oven until golden brown and crisp, about six to eight minutes. Remove the cookies from the oven and allow them to cool completely while you make the icing.
For the frosting, beat the butter in a medium mixing bowl. Slowly add in the icing sugar about 1/4 cup at a time, beating well after each addition, until a smooth buttercream forms. Beat in the salt, vanilla, and enough milk or cream to make the frosting light and fluffy.
Just before serving, fold in the rainbow sprinkles and either serve in a bowl surrounded by the cookies or portion out personal servings of frosting and cookies in small reusable containers. Stored separately, the buttercream and cookies will last for three to four days.
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