Preheat oven to 325F and grease an 8 x 4-inch or 4 x 10-inch loaf pan. Set aside.
In a large bowl with an electric mixer or the bowl of a stand mixer with the paddle attachment, beat together the butter and sugar until light and fluffy, about 3 minutes. Beat in the eggs, one a time, beating well after each addition and scraping down the sides of the bowl as needed. Beat in the vanilla.
In a medium bowl, whisk together the flour, salt, baking soda and baking powder. Add 1/2 of the dry mixture to the butter mixture, then the sour cream, then the rest of the flour mixture, mixing just until combined.
Remove 1 cup of the batter to a medium bowl. Add the cinnamon swirl ingredients to it - the brown sugar, cinnamon and milk and stir to combine.
Spoon a thin layer of the plain batter into the bottom of your prepared loaf pan. Using a spoon, drop alternating spoonfuls of the plain and the cinnamon batter into the pan. Top with the remaining plain batter and smooth the top. Using a long skewer or knife, run through the batter in the pan a few times (either lengthwise or from side to side). Just about 3 lengthwise or 4-5 side to side should do it).
Make topping by combining the brown sugar and cinnamon in a small bowl. Sprinkle over the top of the loaf.
Bake loaf in the preheated oven for 55-65 minutes, or until a knife inserted in the centre of the loaf comes out clean. (4x10 pan will cook more quickly, 55-60 minutes, while an 8x4 will take a bit longer, closer to 65 or a few minutes more).
Allow loaf to cool in the pan about 10 minutes, then run a knife around the outside and remove to a cooling rack to cool.
Be sure to read the "Cook's Notes" in the original post, for more tips, options, substitutions and variations for this recipe!