Preheat the oven to 350 degrees F. Butter a 9-by-5-inch loaf pan
In a large bowl, sift together the all-purpose flour, cake flour, baking powder and salt. Set aside.
In a stand mixer or using a hand mixer, cream together the butter and cream cheese on medium speed until light and fluffy. Add the sugar and mix for 1 minute. Add the eggs, one at a time, and then the vanilla; mix for 1 minute, until well blended. With the mixer on low speed, slowly add in the dry ingredients and mix until completely blended.
Pour the batter into the prepared loaf pan and bake until a toothpick inserted into the center comes out clean, 50 minutes. Let cool for 10 minutes before turning the cake out onto a wire rack to cool completely.