Preheat oven to 425 degrees.
Cut butter into cubes, then freeze until ready to add it.
Use a prepared cast iron skillet or other desired pan. If using a pan other than cast iron, grease then flour the pan.
In a large mixing bowl, add the flour, sugar, salt, baking powder, and baking soda.
Cut butter into the flour mixture, leaving pieces of butter.
In a mixing bowl, beat together the buttermilk, eggs, and vanilla.
Cut remaining butter into small pieces.
In a separate bowl, add remaining butter, brown sugar, and the cinnamon.
Melt the butter and sugar mixture in the microwave. Allow to cool slightly, then pour gently onto a sheet of parchment paper and press and freeze.
Pour buttermilk mixture into flour mixture and stir until just combined.
Once chilled, cut the cinnamon butter into pieces and stir into the dough.
Spread mixture into pan or skillet, then bake for 23-25 minutes.
Cool, cut into eighths, and serve. Store in an air-tight container for up to 4 days.