How to Make It
Preheat oven to 350°F. 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.
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.
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.
Spoon batter into 2 (12-cup) muffin pans lined with paper baking cups, filling three-fourths full (about 1/4 cup per muffin).
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.
Bake at 350ºF 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.