Preheat the oven to 325°F. Grease an 8x4 inch metal loaf pan. Line with a strip of parchment, then grease again and dust with more flour
In a mixing bowl, beat the butter until creamy. add the granulated sugar and beat until fluffy, scraping bowl once or twice. Add the eggs 1 at a time, combining well after each addition and scraping the bowl.
With mixer on lowest speed or by hand, add the flour in 2 parts, stirring until blended. Add the salt and baking soda, stirring well. Add the sour cream, oil, and vanilla extract. Scrape down the sides and bottom of the bowl and mix the batter until just combined. Set the batter aside.
CINNAMON SWIRL: In a small bowl, mix together melted butter, sugar, flour and vanilla.
TO BAKE: Pour ⅓ of the batter into the prepared pan, then drizzle ½ of the cinnamon swirl mixture over it.
Using a butter knife or skewer, swirl the mixture through the cake batter.
Repeat with the rest of the cake batter and cinnamon swirl. Top with the remaining batter.
Bake for 60 minutes, or until a toothpick inserted into the center of the cake comes out mostly clean. A meat thermometer inserted in the center should register about 210 F.
Let the cake cool in the pan on a wire rack for 10 minutes, then invert onto a serving plate. Let cool to room temperature.
To make the frosting, beat the cream cheese and butter until creamy. Stir in the sugar and beat until well blended, then beat in the vanilla. Add milk gradually to make it a drizzling consistency, then drizzle the icing over the cake. If you prefer, put it in a heavy duty freezer bag, push it all down toward the bottom corner, snip off the bottom corner and squeeze it over. You'll have more than enough.
Notes
The original cinnamon swirl filling calls for ⅓ cup unsalted butter, ⅔ cup brown sugar, 1 tablespoon flour, 1 ½ teaspoons cinnamon and 1 teaspoon of vanilla. My measurements are ⅓ of that, but I rounded up slightly to make the measurements less awkward.