Preheat oven to 375°F.

Line 18 muffin cups with cupcake liners; set muffin tins aside.

Using a large bowl, whisk together flour, cocoa powder, baking soda and salt; set bowl aside.

In a separate bowl, whisk together vegetable oil, eggs, sour cream, milk, vanilla extract, granulated sugar and brown sugar.

Pour wet mixture into dry mixture; fold until just combined.

Carefully fold in 1 cup of chocolate chips.

Spoon batter into muffin cups, filling each cup ⅔-¾ full.

Sprinkle tops of muffins with ½ cup of chocolate chips and chopped hazelnuts.

Bake for 18-20 minutes, or until a toothpick inserted into center of a muffin comes out mostly clean.

Meanwhile, use a microwave or double boiler to melt the remaining ½ cup of chocolate chips. (Note: If microwaving, add 1 Tbsp of milk to the bowl.)

Transfer melted chocolate into a piping bag (or a sandwich bag with one corner snipped off).

Once muffins have been removed from oven, drizzle tops with melted chocolate.