For the muffins: Position a rack in center of the oven and preheat to 425 degrees F. Crush the blueberries in a medium bowl with a potato masher or fork until completely smashed. Lightly coat the cups of a standard 12-cup muffin pan with nonstick spray.
Whisk together the all-purpose flour, whole-wheat flour, granulated sugar, baking powder, apple pie spice and salt in a large bowl.
Add the buttermilk, butter, lemon zest, lemon juice, vanilla extract, egg and yolks to the mashed blueberries and stir until combined and creamy; gently fold into the flour mixture until just combined (it's ok if there are some lumps). Divide the batter evenly among the prepared muffin cups.
Bake, rotating the pan halfway through, until a toothpick inserted into the center comes out clean, 25 to 30 minutes. Transfer the pan to a wire rack and let the muffins cool in the pan for 5 minutes. Remove the muffins and let cool completely on the rack.
For the frosting: Meanwhile, beat the cream cheese and butter in a medium bowl using an electric mixer on high speed until light and creamy, about 5 minutes. With the mixer on low speed, gradually add in the confectioners' sugar. Once all the sugar has been added, increase the speed to high and continue to beat until stiff and creamy, about 3 minutes. Transfer to a pastry bag fitted with a 1/4-inch flat tip.
Starting from the top of a cooled muffin, leaving a small space for the eyes, pipe frosting from left to right forming crisscrossing ribbons that mimic the wrapping of a mummy. Pipe a small amount of frosting on the back of 2 blueberries and two candy eyes. Place the blueberries, frosting-side down, onto the unfrosted part of the muffin. Top each blueberry with eyes. Repeat with remaining frosting, muffins, blueberries and eyes.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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