Preheat oven to 350 degrees F. Grease a 9 by 5 by 3-inch loaf pan with butter. In a medium bowl combine the flour, baking powder, and salt. Stir to combine.
Using an electric mixer, cream together the butter, ricotta, and sugar until light and fluffy, about 3 minutes. With the machine running, add the eggs 1 at a time. Add the vanilla and orange zest until combined. Add the dry ingredients, a small amount at a time, until just incorporated. Pour the mixture into the prepared pan and bake until a toothpick comes out clean and the cake is beginning to pull away from the sides of the pan, about 45 to 55 minutes. Let the cake cool in the pan for 10 minutes then transfer to a wire rack to cool completely. Using a mesh sieve, dust the cooled cake with powdered sugar.
Note: To determine if cake is done baking, use an instant-read thermometer to take the internal temperature of the pound cake - it should be about 200 degrees.