Preheat oven to 400 F.
If not using buttermilk, pour 1 cup regular milk (preferrably whole, full-fat milk) into measuring cup and add apple cider vinegar (approximately 1 Tablespoon) and let sit until ready to add to mixture.
Add softened butter to a large bowl cut into small pieces. Add flour, baking powder, salt and sugar and combine with hands until fairly well incorporated (there will be pea-sized pieces of butter).
Add buttermilk to flour mixture and mix with spoon until incorporated.
Take blueberries from freezer and measure out 1 cup full. Add to flour mixture and mix gently with spoon or hands just until blueberries are scattered about.
Dump flour mixture onto floured countertop space. Cover with a bit of flour on all sides, but don't over flour. (You want it easy to handle but you don't want to dry it up with too much flour.)
Roll out gently with rolling pin to about 1 inch thick, taking care not to mash the blueberries.
With a 3-inch round biscuit cutter (or I use a cup or glass), cut out the biscuits, trying not to split the blueberries in the process.
Lightly oil a cast iron skillet.
Place each biscuit in the prepared cast iron skillet, scrunching up and making room so that 10 biscuits will fit as you add more.
Place in the oven. Cook for 30 minutes or until golden brown.
While cooking, prepare the glaze.
Add all glaze ingredients to a medium-sized bowl and blend with a small whisk until smooth.
When biscuits are done, let sit for 5 minutes after removing from the oven.
Add small spoonfuls of glaze to the top of the biscuits (about 5-6). Serve and add more glaze to individual portions.