Pre-heat oven to 325°. Line a loaf pan with parchment paper and set aside.
Mix flour, whey and casein protein powder, baking powder, baking soda, and salt. Set aside.
Add butter and sugar to the bowl of a stand mixer or large mixing bowl. Cream on medium high until the mixture is pale and fluffy, about 3-5 minutes. Turn speed to medium and add eggs one at a time. Next, add yogurt and wait until combined. Then add lemon juice, lemon zest, and vanilla extract. Beat for another minute, then turn off.
Add one third of the flour mixture, then turn mixer on at low speed until almost combined. Add half of the buttermilk until just combined. Repeat this process with another third of the flour mixture and remaining buttermilk. Once it is just combined, add the remaining flour mixture and mix until just combined, being careful not to overwork the batter.
Transfer the batter to a loaf pan and bake at 325° for 1 hr and 15 minutes. Tent the pan very loosely with foil when 20-30 minutes of baking time remain to ensure the top doesn't get too browned. The loaf is done when a toothpick inserted in the center comes out clean.
Note 1: Don't add the foil too early or leave the oven open too long or you'll risk having a collapsed loaf.
Note 2: If making mini-loaves, check for doneness starting around the 1 hour mark.
Allow the loaf to cool in the pan for about 15 minutes before removing. As it cools, whisk together the ingredients for the lemon soak. Brush the top of the loaf while it is still hot in the pan. Once the loaf is removed from the pan, brush it again on all sides with the lemon soak. Set aside to cool completely.
Once the loaf is completely cooled, whisk together icing ingredients and drizzle it over top of the loaf. I like to do this over parchment paper so I don't lose any icing, but you can also do this over a cooling rack so the excess icing drips off.
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