In a large bowl, whisk together pancake mix, salt, and garlic powder. Add cubed butter and flatten and break up pieces of butter and gently work it into the flour mixture a bit. Work quickly to prevent butter from melting.
Add cheese, scallion, and thyme (optional) and mix in until combined. Add buttermilk and fold it into the flour mixture until a craggly dough comes together.
Dump dough onto a floured surface and push together to bring into one cohesive dough. Pat into a 1/2 inch thick rectangle, fold in thirds like a letter, pat down gently, turn 90 degrees, and fold again.
Pat down with hands or rolling pin to a 1 inch thick rectangle and cut into 6 or 8 pieces. (Optionally, use a round cookie cutter to cut out biscuits, gently re-roll excess dough and cut again).
Chill biscuits in the freezer for 15 minutes, meanwhile, PREHEAT oven to 425°F.
Place biscuits onto a parchment lined baking sheet about 1-2 inches apart. Brush tops with egg wash and bake for about 15 minutes, until biscuits are golden brown.