Preheat the oven to 350 degrees and thoroughly grease a 9-inch x 5-inch loaf pan.
To make the cake, in a medium bowl sift together the dry ingredients (flour, baking powder, baking soda and salt).
In a separate bowl, using a hand or stand mixer, beat the shortening and sugar until fluffy (for 1-2 minutes).
Add the eggs, vanilla extract and sour cream into the shortening and sugar mixture and mix until combined. Scrape down the bowl once complete.
Add the dry ingredients to the wet ingredients in two parts, mixing well between each addition. Scrape down the bowl once complete. Set the bowl aside.
To make the cinnamon filling, mix the brown sugar and cinnamon in a small bowl using a fork.
Spread half of the batter throughout the greased loaf pan.
Spread about ¾ of the cinnamon sugar filling over the batter.
Spread the remaining batter over the filling, covering the cinnamon sugar filling completely with batter.
Sprinkle the remaining cinnamon sugar filling over the top, then bake for 50-60 minutes, or until a toothpick comes out clean. Allow the loaf to cool for at least 30 minutes before removing it from the pan and adding the icing.