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1
Preheat oven to 350u00b0F. Spread oat flour on a large rimmed baking sheet. Toast in preheated oven until very lightly browned and fragrant, 10 to 12 minutes, stirring after 5 minutes. Remove baking sheet to a cooling rack, and cool completely, about 20 minutes.
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2
Beat softened butter with a heavy-duty stand mixer on medium speed until creamy; gradually add granulated sugar and 3/4 cup of the brown , and beat until light and fluffy. Add eggs, 1 at a time, beating just until combined after each addition. Beat in orange zest and vanilla just until combined.
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3
Whisk together all-purpose flour, baking powder, baking soda, cinnamon, cloves, nutmeg, 1 1/2 cups of the toasted oat flour, and 1/2 teaspoon of the salt in a medium bowl. Add flour mixture to butter mixture alternately with buttermilk, beginning and ending with flour mixture, beating just until combined after each addition. Gently fold in chopped prunes.
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4
Spoon batter into 2 (12-cup) muffin pans lined with paper baking cups, filling three-fourths full (about 1/4 cup per muffin).
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5
Combine uncooked regular oats and remaining 1/2 cup toasted oat flour, 1/4 cup brown sugar, and 1/4 teaspoon salt in a medium bowl. Cut in cold butter with a pastry blender or 2 knives until mixture resembles very coarse meal. Top each muffin with about 1 tablespoon oat topping.
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6
Bake at 350u00b0F until a wooden pick inserted in center of a muffin comes out clean, 19 to 22 minutes. Remove pans to wire racks, and let cool 5 minutes. Remove muffins to wire racks, and let cool completely or serve warm.
