Instructions:
- Preheat oven to 350°F (175°C). Line a standard 12-cup muffin tin with paper cases.
- Combine Dry Ingredients: Whisk together flour, sugar, cocoa, baking soda, baking powder, salt, cinnamon, and nutmeg in a large bowl until thoroughly combined.
- Combine Wet Ingredients: In a separate bowl, whisk eggs, milk, oil, and vanilla until just combined.
- Mix Batter: Gradually add the wet mixture to the dry mixture. Beat on low speed until just incorporated. Do not overmix.
- Bloom the Cocoa: Carefully pour the boiling water or hot coffee into the batter while mixing on the lowest speed until smooth. The batter will be thin.
- Bake: Divide batter evenly among the 12 liners (about ⅔ full). Bake for 18–22 minutes, or until a toothpick inserted into the centre comes out with moist crumbs attached. Cool completely on a wire rack.
- Prepare Filling: Once cupcakes are fully cool, load the ½ cup of Nutella filling (B) into a piping bag or syringe.
- Core and Fill: Using a corer or small knife, gently remove a plug from the centre top of each cupcake. Inject 1 to 2 teaspoons of Nutella into the cavity of each cupcake. Patch the hole with the removed cake scrap if necessary.
- Make Frosting: Beat the very soft butter until light and fluffy. Gradually beat in the sifted powdered sugar, the ½ cup of Nutella for frosting, and hazelnut extract until smooth.
- Adjust Frosting Consistency: Beat in heavy cream or milk one tablespoon at a time until the frosting reaches a smooth, pipeable consistency.
- Pipe and Serve: Transfer frosting to a piping bag and pipe generous swirls onto the cooled, filled cupcakes. Garnish with crushed hazelnuts if desired.