In a large bowl or the bowl of a stand mixer, beat together the butter and sugar until light and fluffy. Add the eggs one at a time, followed by the vanilla, and mix well.
Add the flour, baking powder, cinnamon, and salt and mix until a firm dough forms. Chill 30 minutes.
Preheat the oven to 375ºF. Roll the dough into one inch balls and place on baking sheets lined with parchment paper. Flatten each ball slightly with your hand or with a drinking glass to be about 1/3- to 1/2-inch thick. Bake 7 to 9 minutes, or until the bottom edges are just slightly golden brown. Remove to a cooling rack and let cool before frosting.
In a large bowl, beat together the cream cheese and butter with an electric mixer until light and fluffy. Beat in the powdered sugar until smooth. Add the vanilla and milk as needed to reach desired consistency. Beat until light and fluffy.
Pipe onto cooled cookies (you can use a 1M tip or snip off the top of a zip-top bag) in a spiral shape. Sprinkle the tops with cinnamon or cinnamon sugar. Store in a covered container up to 3 days.
Freezer instructions: Allow cookies to cool completely. Place on a baking sheet and then freeze 30 minutes. Once the frosting has hardened you can stack in a covered container. Take out 1 hour before ready to serve. Variation: You could also serve these cookies with a cinnamon buttercream or drizzle with a cream cheese glaze.