Preheat the oven to 350 degrees F (175 degrees C). Grease a cookie sheet or line with parchment paper.

In a medium bowl, cream together butter and sugar until light and fluffy. Beat in eggs and vanilla. Sift together flour, cinnamon, baking powder, and salt; mix into egg mixture. Stir in hazelnuts.

Shape dough into two 12-inch long logs; place on the prepared baking sheet and press down to 1/2-inch thickness.

Bake for about 30 minutes in the preheated oven, or until the edges are golden and the center is firm. Remove from oven to cool on the pans. When logs are cool enough to handle, use a serrated knife to slice the logs diagonally into 1/2-inch-thick slices. Return the slices to the baking sheet.

Bake for an additional 10 minutes, turning over once. Cool completely, and store in an airtight container at room temperature.