Line a 12-cup muffin tin with paper liners or spray with nonstick baking spray that contains flour.
In a medium bowl, whisk together flour, baking powder, cinnamon, nutmeg, and kosher salt.
2 1/2 cups all-purpose flour, 2 1/2 teaspoons baking powder, 1/2 tsp ground cinnamon, 1 teaspoon kosher salt, 3/4 tsp ground nutmeg
In a large bowl, whisk together the sugar and eggs until thick and pale, about 1 minute. Slowly whisk in the melted butter until combined. Add eggnog, rum extract and stir until smooth.
3/4 cup granulated sugar, 2 large eggs, 1/2 cup unsalted butter, 1 cup full fat eggnog, 1 1/2 tsp rum extract
Gently fold the dry ingredients and floured berries into the wet mixture until just combined. Do not overmix—some lumps and streaks of flour are okay.
Cover the bowl with plastic wrap and let the batter rest for 15 minutes while you preheat the oven to 425°F (218°C).
Divide the batter evenly among the muffin cups using an ice cream scoop. Sprinkle the tops with coarse sugar. Bake for 15–20 minutes, or until golden brown and a toothpick inserted into the center comes out clean.
Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely. In a small bowl, combine the powdered sugar, eggnog, and nutmeg. Drizzle over the cooled muffins.
1/2 cup powdered sugar, 1-2 tbsp eggnog, 1/4 tsp ground nutmeg