Preheat the oven to 350°F (175°C) and line two baking sheets with parchment paper.
In a medium-sized mixing bowl, combine the all-purpose flour, baking powder, baking soda, ground cinnamon, and salt. Whisk together and set aside.
In a large mixing bowl, cream together the softened unsalted butter, packed brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes.
Add in the egg and vanilla extract, mixing until well incorporated.
Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined.
Scoop out portions of the dough, rolling each into a ball, and place on the prepared baking sheets, flattening slightly.
For half of the cookie rounds, press a small indent in the center and spoon approximately 1/2 teaspoon of jam into each. Place the remaining cookie rounds on top and seal the edges, crimping with a fork.
Bake for 10-12 minutes, or until the edges are lightly golden.
While baking, prepare the icing by combining powdered sugar with 2-3 tablespoons of milk until smooth.
Once cooled, drizzle icing over the cookies and add rainbow sprinkles if desired.
Notes
For an eye-catching display, arrange the iced cookies on a vibrant plate and serve with milk.