Whisk together 3 cups flour, 1 tablespoon baking powder, and 1 teaspoon salt in a medium bowl. Beat together 1-1/2 sticks butter and 3/4 cup sugar in a large bowl on medium speed until light and fluffy, about 2 minutes. Add 1 egg, 1 egg yolk, 1/4 cup sour cream, 1 tablespoon vanilla and the green food coloring; beat until combined well. Reduce speed to low and gradually add flour mixture, beating just until combined.

Transfer to a lightly floured surface and knead until dough just holds together. Flatten into a disk, wrap well in plastic wrap and chill until firm, about 2 hours. Repeat, making a second batch of dough using the red food coloring.

Preheat oven to 350 degree F. Lightly grease 4 large cookie sheets with butter.

Unwrap 1 disk and cut in half (rewrap remaining half and keep chilled until ready to use). Roll dough on a lightly floured surface with a floured rolling pin until 1/8 inch thick. Using two maple (3 inches and 2-2/3 inches in height) and two oak (3-3/4 inches and 3-1/4 inches in height) leaf cookie cutters, cut out cookies. Press together scraps of dough and flatten into a disk; wrap in plastic wrap and chill until firm, about 20 minutes. Beat together remaining egg and 1 teaspoon water; brush over cookies. Transfer to prepared cookie sheets, placing 1 inch apart.

Bake cookies, 2 sheets at a time, in upper and lower thirds of oven, switching positions halfway through, until pale golden, 10 to 12 minutes. Transfer cookies on sheets to racks and cool until firm, about 2 minutes. Using a metal spatula, transfer cookies to racks and cool completely.

Repeat with remaining dough disk and dough.

Make fondue: Microwave chocolate-hazelnut spread in a microwave-safe bowl on medium high 1 to 2 minutes or until softened, stirring halfway through. Transfer to a serving bowl and place on a large platter. Arrange leaves in a wreath shape around bowl of fondue and garnish with hazelnuts. Makes about 10 dozen leaves.