Preheat the oven to 350°F. Grease a 9 by 5-inch loaf pan.

In a large mixing bowl, sift together the cake flour, all-purpose flour, baking powder, and salt. Set aside.

In the bowl of a food processor or high-speed blender, combine the sugar and lemon zest. Pulse a few times to evenly distribute the zest. Add the lemon juice, eggs, and sour cream and pulse until combined. On low speed, gradually drizzle in the melted butter in a slow steady stream until well combined.

Make a well in the dry ingredient mixture. Pour in the wet ingredients. Mix gently with a rubber spatula until just combined. Batter will be slightly thin.

Pour mixture into prepared pan and spread evenly. Bake until a toothpick inserted into the center comes out clean or with just a few moist crumbs attached, about 50 minutes if using a metal pan, or once an internal temperature of 200-205°F is reached. If using a glass or ceramic pan, decrease your baking temperature to 325°F and increase your baking time by about 10 minutes.

Cool in pan for 15 to 20 minutes then run a thin flexible knife around the edges before inverting onto a wire rack to cool completely.

In a small bowl, whisk together the glaze ingredients until thick but pourable. Add more juice or milk to thin out or more sugar to thicken to your desired consistency. Spoon over the cooled loaf, letting it drip down the sides if desired. Let set for 15 minutes before slicing and serving.