Step 1
Whisk 7 cups (875 g) all-purpose flour, ¼ cup (50 g) sugar, 2 Tbsp. plus 2 tsp. baking powder, 1 Tbsp. plus 1 tsp. garlic powder, 1 tsp. baking soda, 2 oz. Parmesan, finely grated (about 1 cup), ½ cup (packed) finely chopped parsley, 2 Tbsp. Diamond Crystal or 1 Tbsp. plus ½ tsp. Morton kosher salt in a large bowl to combine. Add sliced 2 cups (4 sticks) chilled unsalted butter, thinly sliced; toss to coat, then smash into smaller, flatter pieces (about the size of a dime) with your hands. Chill at least 15 minutes, or cover and chill up to 12 hours.
Step 2
Drizzle 2½ cups buttermilk over butter mixture; toss with a fork to incorporate, then knead lightly in bowl until a shaggy dough forms (it will look dry). Turn dough out onto a surface; pat into a 1″-thick square, gently pushing dry bits into dough as best as possible. Roll slightly to flatten to about ½” thick. Using a bench scraper, cut dough into 3 equal pieces. Stack pieces on top of one another, sandwiching any loose dry bits of dough between layers; press down with your hands to flatten and pat into a rectangle. Tidy edges with bench scraper. Roll dough into a 1″-thick rectangle. (If dough is sticking to rolling pin, scrape off with bench scraper and press back into larger block.) Repeat cutting, stacking, and rolling process 2 more times (dough will look less dry as you move to the final stage), making sure final rectangle is 1″–1¼” thick. Wrap tightly in plastic and chill at least 1 hour and up to 12 hours.
Step 3
Place a rack in middle of oven; preheat to 425°. Unwrap dough and trim ⅛” off sides with a knife to tidy edges (this will also help biscuits rise later in the oven). Cut dough into 4 equal rectangles, then cut each rectangle into 6 squares to make 24 biscuits total. Arrange on a parchment-lined baking sheet, spacing about ¼” apart. Chill 30 minutes, then bake until tops are golden brown, 35–40 minutes.
Step 4
Just before the biscuits are finished baking, heat 4 garlic cloves, finely grated, a large pinch of salt, and remaining 4 Tbsp. (½ stick) unsalted butter in a small saucepan over medium until butter is melted. Remove from heat and stir in remaining 3 Tbsp. finely chopped parsley.
Step 5
Remove biscuits from oven. Brush garlic butter over; sprinkle lightly with Parmesan.