In the bowl of a stand mixer fitted with a paddle attatchment, add the butter and sugar. Cream on medium speed for about 5 minutes, pausing 1/2 way through to scrape the sides of the bowl with a nonstick spatula. Add in the vanilla and mix, scrape down the sides of the bowl again
Add the flour, salt, and cinnamon to the wet ingredients and mix until a dough forms. Place dough on a clean sheet of parchment paper and place another sheet of parchment on top. It will be slightly sticky so you will use a rolling pin to roll out the dough in between these two parchment sheets. Roll the dough into a rectangle that it about 1/4 inch thick. If this is too thick you won't have nice swirls in the dough! Place the rolled out dough (still in the parchment) in the fridge to firm up for 15-20 minutes while you make the filling.
In a separate small bowl, add the butter, sugar, cinnamon and salt. Mix with a wooden spoon until smooth and thoroughly combined. Remove the chilled rolled out dough from the fridge. Using an offset spatula, carefully spread the filling on top of the dough. Try to get this layer as smooth and even as possible. Roll the dough into a very tight log, starting at the long end of the rectangle. Wrap the rolled log in cling wrap and let chill in the fridge overnight or at LEAST 4 hours. This is key to making sure the filling doesn't melt out!
When dough is chilled, preheat oven to 350 F. Line two baking sheets with parchment paper.
Unwrap the dough and place on a clean work surface. Slice the log into 1/4 inch thick slices, you will need a good sharp knife for this! Place the cookies onto the prepared baking sheets leaving at least 2-inches in between each cookie apart. Bake for 18 to 20 minutes. Let the cookies cool completely.
