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Line a baking sheet with parchment. Place a rack in the upper third of your oven. Preheat the oven to 400°F.
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Measure the butter, keeping it in stick form. Put it in the freezer while you combine the dry ingredients.
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In a large bowl, whisk together the flours, baking powder, garlic powder, and salt until thoroughly combined. Place a standard box grater in the large bowl on top of the dry ingredients.
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Remove the butter from the freezer and grate it using the side with large holes, letting the strands of butter fall into the dry ingredients below.
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Use your hands to lightly toss the butter into the dry ingredients to ensure each piece is completely coated in the flour. Place the bowl into the refrigerator or freezer to keep the butter and flour cold while you combine the rest of the ingredients.
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Using a high-speed blender, food processor, or immersion blender, combine the buttermilk, spinach, parsley, basil, and mayonnaise. Blend until the liquid is vibrant green, with only small flecks of herb throughout. This will take 20 to 30 seconds in a high-speed blender or up to 1 minute if using a food processor or immersion blender.
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Remove the butter/flour mixture from the refrigerator, add the chopped chives, and toss together with a flexible spatula. Create a well in the center of the dry ingredients and pour in the blended buttermilk. Quickly but gently, stir the flour and buttermilk together, being sure to incorporate any dry flour that might be hidden at the bottom of the bowl. The mixture should be shaggy but cohesive.
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Lightly oil a 1/3-cup measure or a popover scoop and begin to scoop the batter into 8 biscuits onto the prepared pan, leaving 2" to 3" between each one. If using 1/3 cup, round the scoops; if using a popover scoop, make them level.
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Place the pan with the scooped biscuits in the freezer for 20 to 30 minutes, if time allows, which will help them hold their shape in the oven.
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Use a pastry brush to coat the top of each biscuit with egg wash, then sprinkle with a pinch of black pepper.
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Bake the herb biscuits for 10 minutes, then lower the heat to 350°F and bake for another 8 to 10 minutes. The biscuits are done when they’re firm to the touch, beginning to turn golden, and lightly browned on the bottoms.
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Storage information: Herb biscuits can be stored in an airtight container at room temperature for several days. To reheat, place in a 300°F oven for 5 to 10 minutes or microwave in 15-second increments until warm.
