Preheat Oven & Prepare Pan: Preheat oven to 350°F (175°C). Grease a 9x5-inch loaf pan with non-stick cooking spray or butter, ensuring all sides are coated.
Combine Dry Ingredients: In a medium bowl, whisk together 1 cup all-purpose flour, 1 teaspoon baking soda, and ¼ teaspoon sea salt. Set aside.
Mix Wet Ingredients for Banana Bread: In a large mixing bowl, combine ¾ cup sugar and ½ cup softened butter. Beat with a hand mixer or stand mixer on medium speed until light and fluffy (2–3 minutes). Add ¼ cup milk, 1 teaspoon vanilla extract, and 2 eggs, mixing until fully incorporated. Gradually fold in the dry ingredient mixture using a spatula until just combined.
Incorporate Bananas: Add 3 mashed bananas to the batter, stirring gently until evenly distributed. Avoid overmixing; small lumps are acceptable.
Prepare Cream Cheese Filling: In a separate bowl, beat 8 ounces softened cream cheese, 1 egg, 2 tablespoons sugar, and 2 tablespoons flour with a mixer on medium speed until smooth and creamy (1–2 minutes).
Layer Batter & Filling: Pour half of the banana bread batter into the prepared loaf pan, spreading evenly. Spoon all of the cream cheese filling over the batter, ensuring it covers the surface. Top with the remaining banana batter, spreading gently to seal the edges. Optional: Sprinkle ¼ cup chopped walnuts over the top.
Bake: Bake for 50–60 minutes, or until a toothpick inserted into the center comes out clean (avoid testing the cream cheese layer). If the top browns too quickly, tent loosely with foil.
Cool & Serve: Let the loaf cool in the pan for 10 minutes. Carefully transfer to a wire rack to cool completely (about 1 hour) before slicing. Store leftovers as directed.