Add flour, baking powder, and salt to a small bowl mixing to combine then set aside.
In the bowl of a stand mixer, fitted with paddle attachment, add the butter and mix on high for 2 minutes.
Add the sugar into the whipped butter and mix again on high speed for about 1 minute.
Add the egg, and vanilla to the butter and then beat on high speed until it is fully combined.
Now get the small bowl with the flour mixture prepared earlier, and add it into the butter mixture. Mix again on medium low, scrapping sides of the bowl as needed, until combined.
Divide the dough in half. Working one half at a time place the dough onto parchment (or a silicone mat) then roll into a rectangle, about 10 X 7 inches.
Cover each rectangle with the softened butter, 1 tablespoon per rectangle, then prepare the cinnamon sugar filling by combining the ingredients into a small bowl. Sprinkle the cinnamon sugar mixture evenly over the tops of both rectangles.
Working one at a time roll the cookie up tightly, fixing any cracking with hands as you roll, so that you have a 10" cookie log. Wrap the log into plastic wrap then place in the fridge for 2 hours to chill, repeat this process with the second half.
Near the end of chilling preheat the oven to 350 degrees and line cookie trays with parchment.
Remove the chilled dough from the fridge and slice into 1/2" rounds. Place the sliced cookies onto prepared baking sheets, then place in the preheated oven to bake. About 10-12 minutes.
Once cookies are baked remove from the oven and allow to cool on the tray for 5 minutes before removing to a wired rack to cool completely before icing.