Toast hazelnuts in a 10- by 15-inch baking pan in a 350° regular or convection oven until golden beneath skins, 7 to 10 minutes. Pour onto a clean linen towel. When nuts are cool enough to handle, rub in towel to remove loose skins. Whirl hazelnuts in a food processor until finely chopped, or finely chop with a knife; you should have 1 1/4 cups. Reserve 1/4 cup to sprinkle over cooked pancakes.
In a bowl, mix all-purpose flour, whole-wheat flour, sugar, cinnamon, baking powder, baking soda, and salt. In a small bowl, whisk eggs, buttermilk, and butter until blended. Stir egg mixture into flour mixture until evenly moistened, then gently stir in remaining 1 cup hazelnuts.
Place a nonstick griddle or a 12-inch nonstick frying pan over medium heat (350°). When hot, coat lightly with oil and adjust heat to maintain temperature. Spoon batter in 1/3-cup portions onto griddle and cook until pancakes are browned on the bottom and edges begin to look dry, about 2 minutes; turn with a wide spatula and brown other sides, 1 1/2 to 2 minutes longer. Coat pan with more oil as necessary to cook remaining pancakes.
Serve pancakes as cooked or keep warm in a single layer on baking sheets in a 200° oven for up to 15 minutes. Sprinkle with reserved hazelnuts.