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Preheat the oven to 425°F.
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To make the biscuits: Combine the flour and cheese powder in a large bowl.
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Work in the cold butter until it's unevenly crumbly, with the largest crumbs about the size of large peas.
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Stir in the buttermilk; the mixture will begin to hold together on its own, and leave the sides of the bowl. Add more buttermilk a tablespoon at a time, if necessary.
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Transfer the dough to a well-floured work surface, and fold it over onto itself 3 to 5 times.
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Roll the dough into a rectangle 1/2" to 3/4" thick.
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Cut out 2 1/2"-round biscuits, patting the scraps together and re-rolling them to make as many biscuits as possible.
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Arrange the biscuits 1" apart on an ungreased or parchment-lined baking sheet (the parchment makes for easy cleanup), and brush with the melted butter.
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Bake the biscuits for 12 to 15 minutes, until they're golden brown.
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Remove the biscuits from the oven, brush with any remaining melted butter, and cool for 10 minutes.
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Split the biscuits in half, and spread a thin layer of red pepper jelly on each half. Layer the arugula and ham on the bottom half of each biscuit and cover with the top half.
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Serve warm with extra red pepper jelly.
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Yield: about a dozen 2 1/2" biscuits.