Preheat your oven to 160FAN or 350 degrees Fahrenheit. Butter and flour a standard loaf pan.
Prepare your apple mixture first. Simply add the apples, butter and brown sugar to a pan or pot and heat over medium heat until the butter has melted and the brown sugar has dissolved. Continue to cook until the apples are softened, but not mushy, about 6-8 minutes. Remove the mixture from the heat and set aside. NOTE: I usually use 2 apples, but you can use up to 4, depending how apple-y you want your loaf.
Next, prepare your cinnamon swirl mixture by whisking or stirring the cinnamon and sugar together completely, eliminating any lumps.
Make the cake batter by first combing the ingredients listed under "Dry Cake Ingredients" in a large mixing bowl.
In another mixing bowl, prepare your wet ingredients: begin by adding the sour cream and vanilla to the bowl and whisking them together. Add the beaten egg and the sugar and whisk to incorporate everything completely.
Add the browned butter and whisk until it has incorporated completely - since butter is a fat, it will want to sit on top of the mixture, not mix right in. Just whisk it until it combines with the other ingredients, it won't take too long.
Add the wet ingredients to the dry ingredients and begin to stir. Then slowly add in the milk, while stirring, and continue to stir everything together until all of the ingredients are thoroughly combined.
Add half of the batter to the loaf pan and use a spoon or small offset spatula to distribute the batter into an even layer.
Add half of the cinnamon swirl directly on top of the batter and use a butter knife to swirl the mixture throughout the batter.
Add the apple mixture directly on top of the batter/cinnamon swirl mixture. Add 1 tbsp of the apples' cooking liquids directly on top of the apples.
Add the remaining batter to the pan and once again be sure to evenly distribute the batter.
Add the remaining cinnamon swirl to the top of the loaf in a single, even layer.
Pop the loaf into the oven and bake for 45-50 minutes, or until a toothpick or knife inserted into the center of the loaf comes out clean. If your loaf browns on the top quickly, just cover it with foil while the inside of the loaf continues to bake.
Allow the loaf to cool for at least 20 minutes on a wire rack before removing it from the pan, preferably close to half an hour.
I like to serve mine with a simple glaze of butter, powdered sugar and maple syrup or just some honey butter. Enjoy hot or at room temp!