In a bowl, combine the flour, baking powder, and salt. Set aside.
In a stand mixer fitted with a paddle attachment (or using a bowl and a hand mixer), beat the butter, sugar, and vanilla extract until light and fluffy.
Add the egg and beat until fully incorporated.
Add the dry ingredients and mix on low speed just until no dry flour is visible.
Wrap the dough in plastic wrap, shaping it into a square. Refrigerate for 10–20 minutes, or until it firms up slightly.
Place the dough on a floured surface and roll it out into a 11 inch by 11 inch (28cm by 28cm) square approximately ¼ inch (5mm) thick. If cracks appear, gently press the dough back together.
In a small bowl, combine all the cinnamon filling ingredients until they form a paste-like consistency.
Spread the filling over the dough, leaving a ¼ inch (5mm) border at one end untouched.
Starting from the side with the filling, roll the dough into a log, ending at the border without filling. Gently press the edge into the roll to seal it.
Wrap the log in plastic wrap and refrigerate until firm enough to slice (about 1 hour).
Preheat the oven to 355F (180C) and line two baking sheets with parchment paper.
Trim the ends and slice the log into ½ inch (1.2cm) thick rounds. You should have at least 20-22 cookies Place the slices on the prepared baking sheets, leaving some space between them.
Bake in two batches for about 11 minutes, or until the edges start to turn golden. Keep the second batch in the fridge while the first batch bakes.
Remove the cookies from the oven and let them cool on the baking sheet for 5 minutes. Then transfer them to a cooling rack to cool completely.
Combine all the icing ingredients in a bowl. Drizzle the icing over the completely cooled cookies and let it set at room temperature.