Preheat oven to 425°F. Grease a 10-inch cast iron skillet generously with butter. Set aside.
In a large bowl, whisk together the flour, baking powder, baking soda, sugar, and salt. Set aside.
Use a box grater to grate the frozen butter into small pieces. You can also cut it into small cubes.
Use your fingers or a pastry blender to cut the butter into the flour mixture until it is the consistency of corn meal. When you squeeze the butter/flour mixture together it should hold together because the flour is coated with the butter. You want some pieces of butter to stay whole. Pro tip: If the mixture gets too warm and the butter starts to melt, pop it into the freezer for about 15 minutes.
Add the buttermilk and thyme leaves, stir using a Danish dough whisk or wooden spoon just until a shaggy dough starts to form. Pro tip: You don’t want to over work it, so it is okay if some of the flour mixture isn’t entirely combined.
Pour the contents of the bowl onto a lightly floured work surface and use a bench scraper or your hand to push the dough together into a rough rectangle. Push the floury bits into the dough, but don’t try to knead them in or the dough will become tough. Pro tip: I like to use the bench scraper because my hands get warm and melt the butter as I am working.
Fold #1. Lightly flour the surface you’re rolling on and the top of the dough. Use a rolling pin to create a 3/4-inch-thick rectangle (the dimensions don’t matter at this point). Use the bench scraper to help lift the short end of the dough, fold 1/3 of the dough into the center. Fold the other 1/3 over the top, like a letter.
Fold #2 & #3. Repeat above step two more times. Pro tip: Each time the dough will become a little neater but don’t worry about that or you’ll be tempted to over work it. This whole process is very easy and gentle, no forcing the dough.
After the fold #3, cut 1/3 of the dough off the short end. On a lightly floured surface, using a rolling pin, roll it into a 9-inch round disk (about 1/4 -inch-thick) and place it in the prepared pan.
Roll the remaining piece of dough into a 1-inch-thick rectangle. Dust the top lightly with flour and use a 2-inch round biscuit cutter to stamp out the dough.
Pro tip: Stamp straight dough and do not twist. Make sure the cutter is well floured so it doesn’t drag through the dough, or you will not get the rise you want. Pro tip: You can press the scraps together to stamp more biscuits, just don’t try to knead them together. They should be sticky enough to just press them together. In a medium sized bowl, stir together the blueberries, sugar and zest. If using fresh berries, you may need to add a teaspoon of lemon juice to get the sugar to cling to the berries. Pour the berries over the disk of dough in the pan. Cover the berries with the 2-inch biscuits.
Brush the tops of the biscuits with melted butter and generously sprinkle with sugar.
Place the cast iron pan on a baking sheet. Bake the biscuits in the middle of the oven for 30 minutes. If the biscuits are browning too fast, loosely tent them with a sheet of foil and continue baking for another 10 to 15 minutes. Test the center of the pan to make sure the biscuits in the middle are baked through. (Pro tip) lift the biscuit up and check the underside, if it isn’t fully baked, return to the oven for another 10-15 minutes with the foil tent. If you used frozen berries this can take close to an hour to bake.
You can serve the biscuits hot from the oven with the berries and ice cream or wait until it is room temperature. Pro tip: If there are leftovers, reheat in the microwave.
Pro tip: You can make this the night before and refrigerate the assembled pan to bake fresh in the morning for a brunch. If you do this chilling technique, you will likely need to add even more time to the bake, up to an hour!