Preheat your oven to 425°F (220°C) and set aside a 9-inch cast iron skillet.

Grate or dice the frozen butter. Place the butter back in the frozen until step 4.

Next, sift the flour and baking powder into a large bowl. Then, add the sugar, salt, and lemon zest. Whisk the ingredients together until combined.

Next, remove the butter from the freezer and add the butter pieces to the flour mixture. Use a pastry cutter or fork to cut the butter into the flour until the mixture resembles coarse, pea-sized crumbs. Alternatively, a food processor will work as well.

Next, push some of flour mixture to the edges of the bowl to resemble a well. Next, gradually add the buttermilk to the center of the well, stirring gently until the dough almost comes together. Next, add your blueberries. Then, continue to stir your dough until and most of the flour is combined and the blueberries are evenly distributed throughout the dough. Do not overwork the dough during this step. The dough should be a bit crumbly, yet moistened.

Dump the dough into the cast iron skillet. With a rubber spatula or floured hands, press the dough evenly into the skillet. Next, cut the dough into 8 pieces like a pizza pie, but do not separate. This step is intended to serve as a score for the biscuits, which will make it easier to cut when they are done.

In a small bowl, beat an egg to make an egg wash for the biscuits. Next, brush the tops of the biscuits with the egg wash.

Bake the biscuits for about 22-27 minutes or until the center of the dough is completely baked through. Then, remove the biscuits from the oven and brush them with butter. Set the biscuits aside for about 15 minutes so they can cool.

While the biscuits are cooling, make your icing. Whisk together the sifted powdered sugar, vanilla, lemon juice, and buttermilk in a medium bowl until a thick icing is formed. If you want your icing thinner, you can add additional buttermilk (note, add about a teaspoon at a time to ensure you don't add too much). Drizzle your icing on your biscuits, cut, and enjoy!