Preheat oven to 350°F (175°C) and line a 6-count muffin pan with muffin liners.
Prepare the flax egg: combine the ground flax seeds and water in a small bowl. Let it sit about 5 minutes, while you prepare the batter.
To a large mixing bowl, whisk together the almond flour, oat flour, white rice flour, baking powder, cinnamon, ginger, nutmeg, salt, and coconut sugar.
Add the pumpkin puree, almond milk, and oil, and mix using a wooden spoon until fully combined. You should get a thick batter.
Fill each muffin liner with about 2 tablespoons of the batter. Remove your hazelnut filling from the refrigerator and scoop out balls of about 1 and 1/2 tsp. Place each ball in the center of the liners, on top of you first layer of batter.
Cover the hazelnut filling with more batter, making sure the filling is fully covered on top and sides.
Bake for 25-27 minutes, or until muffins have a golden brown top. Remove from the oven and let them cool at least 20 minutes before eating. The chocolate filling will be melty and slightly sticky like a nut butter.
These muffins will keep for up to 3 days at room temperature in an airtight container.