1. Preheat oven to 350 F. Line a large baking sheet with parchment paper or a silicone baking mat. Set aside.
2. Sift together flour, baking powder and salt; set aside. Put butter and 1 1/2 cups sugar in the bowl of an electric mixer fitted with the paddle attachment. Mix on medium speed until pale and fluffy, about 3 minutes. Mix in eggs, one at a time. Reduce speed to low and gradually mix in flour mixture.
3. Stir together cinnamon and remaining 2 tablespoons sugar in a small bowl. Shape dough into balls, I used a heaping teaspoon of dough. Roll balls in cinnamon and sugar. Place cookie dough balls on prepared baking sheet and bake for 8-10 minutes, until edges are golden. Let cookies cool on baking sheet for 3 minutes and then transfer to a wire cooling rack. Let cookies cool completely before filling.
4. To make the Biscoff cinnamon filling: Combine the butter, Biscoff Spread, confectioners' sugar, and cinnamon in the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed until smooth. Add the milk and beat until fluffy.
5. To assemble the cookies, find two cookies that are similar in size. Using a knife, spread about a tablespoon of the Biscoff filling on the inside of one cookie. Sandwich the cookies together.