► Heat oven to 425°F.
► Warm butter in a small saucepan over medium-low heat, stirring as it melts and darkens around the edges. As soon as melted butter is brown throughout, about 5 minutes, remove from heat, stirring as butter continues to brown. Stir in chopped sage leaves and let cool.
► In a separate bowl, whisk together flour, baking powder, and salt. Stir in butter, cheese, and milk until combined.
► On a floured surface, roll dough until about ¾ inch thick. Using a biscuit cutter or floured knife, cut into 12 even biscuits and place on a baking sheet lined with parchment paper.
► Beat egg with one tablespoon water. Gently press one whole sage leaf onto top of each biscuit, then paint each biscuit top with egg wash, covering leaves. Bake until tops are golden brown, about 15 minutes.