Preheat oven to 350°F (175°C), grease and line a loaf pan. For the filling, beat cream cheese, sugar, egg, and vanilla until smooth. In another bowl, cream butter and sugar, then add eggs one at a time. Mix in lemon zest, juice, vanilla, and sour cream. In a separate bowl, whisk flour, baking powder, soda, and salt; add to wet mix just until combined. Pour half the batter in the pan, spread filling over, then top with remaining batter and smooth. Bake 55–70 min or until a toothpick comes out clean. Cool completely before slicing