Step 1In the bowl of a standing mixer, beat eggs until lemon-colored. Beat in sugar, oil and Cognac. Gradually stir in the baking powder and flour, mixing on low speed until a soft dough is formed. Add an additional tablespoon or two of flour if necessary.
Step 2Using a rubber spatula, transfer the dough to a large piece of waxed or parchment paper and form it into a fat cylinder shape. Close up the corners and chill until firm, at least 2 to 3 hours (can be chilled overnight).
Step 3Heat the oven to 350 degrees, and line 2 to 3 cookie sheets with parchment paper.
Step 4Divide the dough into 3 pieces. Put 1 section on a lightly floured surface, and chill the others. Knead lightly just until the dough is smooth enough to roll out. Lightly flour a rolling pin and roll out the dough to a scant one-fourth-inch thick.
Step 5Using a 2 1/2-inch round cookie cutter or a glass with a thin rim, cut rounds, and place them on the prepared cookie sheets. Place a rounded half-teaspoon of Nutella in the center of each. Pinch the sides together to form a triangle, leaving the filling exposed in the center. Pick up scraps and form another ball. Repeat until all dough is used.
Step 6Transfer to the prepared cookie sheet, spacing the cookies about an inch apart on the sheet and bake until just lightly browned, 12 to 15 minutes. Transfer to a wire rack to cool.