Preheat oven to 325F with racks in the center and top third.

Beat sugar and butter until well blended.

Beat in eggs one at a time until just blended.

Mix in orange peel, baking soda, and salt.

Add 3 cups of flour, hazelnuts, and dried cherries; stir until well blended.

Add remaining 3 cups flour, 1 cup at a time, stirring until well incorporated.

Transfer dough to floured work surface.

Divide into 4 equal pieces.

Knead each piece until dough holds together well.

Form each piece into a 9-inch-long by 3-inch-wide log.

Place 2 logs on each of 2 ungreased baking sheets, spacing 3 inches apart.

Bake until golden and firm, about 55 minutes, switching and rotating sheets halfway through.

Cool logs on baking sheets for 15 minutes.

Maintain oven temperature.

Transfer logs to a cutting board using a spatula.

Cut warm logs crosswise into 1/2-inch-thick slices using a serrated knife.

Arrange slices cut side down on 2 baking sheets.

Bake biscotti for 10 minutes.

Turn biscotti over and bake until light golden, about 10 minutes longer.

Transfer to racks and cool completely.

Melt chocolate in a bowl over a saucepan of boiling water until smooth.

Remove from heat.

Dip one cut side of each biscotti into melted chocolate to about 1/4-inch depth.

Shake off excess chocolate.

Place biscotti, chocolate side up, on baking sheets.

Refrigerate until chocolate is firm, about 35 minutes.

Dip a pastry brush in cocoa and lightly brush over the chocolate on each biscotti.

Store in airtight containers up to 4 days, or freeze up to 3 weeks.