Preheat your oven to 350°F (175°C) and grease a loaf pan.
In a mixing bowl, combine the flour, baking powder, and salt. In another bowl, beat the butter and sugar until fluffy. Add eggs, milk, lemon juice, lemon zest, and vanilla. Mix until combined, then add the dry ingredients.
In a separate bowl, mix the cream cheese, sugar, egg, and vanilla until smooth.
Pour half of the lemon batter into the prepared loaf pan. Add the cream cheese mixture and top with the remaining lemon batter. Swirl with a knife.
Bake for 50 minutes or until a toothpick comes out clean. Let cool before slicing.
Notes
For added flavor, you can drizzle a lemon glaze on top after baking.