Step 1: Preheat and Prep
Preheat your oven to 350°F (175°C). Grease and flour a 9×5-inch loaf pan or line it with parchment paper.

Step 2: Make the Cream Cheese Mixture
In a medium bowl, beat the cream cheese with sugar, egg yolk, and vanilla until smooth and creamy. Set aside.

Step 3: Make the Loaf Batter
In a separate bowl, whisk together flour, baking powder, baking soda, and salt. In a large mixing bowl, beat the butter and sugar until light and fluffy. Add eggs one at a time, then vanilla. Mix in the sour cream. Gradually add the dry ingredients to the wet ingredients and mix until just combined. Fold in the diced peaches.

Step 4: Layer and Swirl
Pour half of the batter into the prepared pan. Spoon the cream cheese mixture on top, then add the remaining batter. Swirl gently with a knife for a marbled effect.

Step 5: Bake
Bake for 55–60 minutes or until a toothpick inserted into the center comes out clean. Let cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.