Preparation

Make the waffles:

• Heat a waffle maker.
• Put the flour, cocoa powder, hazelnuts, baking powder, baking soda, and salt in a food processor and process until the nuts are finely chopped, about 30 seconds.
• In a large bowl, whisk the sugar and eggs until thick and the sugar begins to dissolve, about 1 minute. Whisk in the butter, sour cream, and vanilla until smooth. Whisk in 3/4 cup warm water until smooth. Add the flour mixture and whisk to combine. The batter will be a little lumpy.
• Cook the batter in the waffle maker according to the manufacturer’s instructions. For crisper waffles, heat the oven to 200°F and set the waffles in a single layer directly on the oven rack.

Make the syrup:

• While you cook the waffles, cook the butter in a heavy-duty 1-quart saucepan over medium-high heat until melted and lightly browned, 2 to 3 minutes. Remove the pan from the heat and whisk in the brown sugar, hazelnut liqueur, and salt until smooth. Return the pan to medium-high heat and bring to a boil. Cook until the mixture begins to foam slightly, about 30 seconds.
• Serve the waffles with the syrup.