Cheddar and Scallion Cheese Skirt Biscuits

Ingredients

• 280g (2 1/4 c) all-purpose flour
• 1 1/4 tsp kosher salt
• 2 tsp granulated sugar
• 2 tsp baking powder
• 1/4 tsp baking soda
• 1/2 tsp mustard powder
• 1/2 tsp garlic powder
• 113g (1/2 c) Ontario unsalted butter, cold and cut into 1/2-inch cubes
• 165g (1 2/3 c) Ontario sharp cheddar cheese, grated and divided
• 1/4 c scallions or chives, finely chopped
• 180g (3/4 c) Ontario buttermilk, cold, plus more for brushing
• Flaky salt, for garnish