Ingredients

Biscotti

1/2 cup unsalted butter (4 ounces)
2 1/2 cups all-purpose flour (about 10 5/8 ounces), plus more for hands
1 1/2 teaspoons baking powder
1 1/2 teaspoons ground cinnamon
1/2 teaspoon kosher salt
1/2 teaspoon ground nutmeg
1/4 teaspoon ground allspice
1/4 teaspoon ground cloves
1/2 cup granulated sugar
1/2 cup packed light brown sugar
2 large eggs, at room temperature
1 1/2 teaspoons rum extract (such as McCormick)

Glaze

1 cup powdered sugar (about 4 ounces)
1/8 teaspoon kosher salt
1/4 teaspoon rum extract (such as McCormick)
2 to 3 tablespoons whole milk, as needed

Directions

Make the biscotti:

Melt butter in a small saucepan over medium-low. Continue cooking, stirring constantly, until butter smells nutty, is browned, and is foaming, and any sizzling has subsided, 6 to 8 minutes. Immediately transfer butter to a small heatproof bowl, scraping up and adding any browned bits from pan. Let cool completely at room temperature, about 45 minutes. Transfer to refrigerator, and chill until firm but not completely hardened, about 30 minutes.
Whisk together flour, baking powder, cinnamon, salt, nutmeg, allspice, and cloves in a medium bowl; set aside. Beat browned butter, granulated sugar, and brown sugar with a stand mixer fitted with the paddle attachment on medium speed until light and fluffy, about 3 minutes. Add eggs, 1 at a time, beating well after each addition and stopping to scrape down sides of bowl as needed, about 30 seconds. Beat in rum extract on low speed until combined. Add flour mixture; beat on low speed until well combined, about 20 seconds. Divide dough in half; wrap each piece tightly in plastic wrap. Chill until firm, about 1 hour.
Preheat oven to 350°F. Line a baking sheet with parchment paper; set aside. Unwrap 1 dough half on a clean work surface. Using lightly floured hands, shape dough into a flattened log, about 10 inches long, 2 inches wide, and 1 inch thick. Place dough log on prepared baking sheet. Repeat process with remaining dough piece, spacing logs at least 3 inches apart. Bake in preheated oven until light golden brown on edges, about 25 minutes. Remove from oven. Let cool on baking sheet 10 minutes. Reduce oven temperature to 300°F.
Carefully transfer baked logs to a cutting board. Using a very sharp serrated knife, cut logs into 1/2-inch-thick slices on a slight diagonal. (It’s important to do this step while biscotti is still warm so that it doesn’t fall apart when slicing.) Return sliced biscotti, cut side down and spaced about 2 inches apart, to baking sheet. Bake at 300°F for 12 minutes. Remove from oven. Flip biscotti over, and return to oven. Bake at 300°F until biscotti are dry, 10 to 13 minutes. Remove from oven. Let cool on baking sheet 10 minutes. Transfer to a wire rack, and let cool completely, about 30 minutes.

Make the glaze:

Whisk together powdered sugar, salt, and rum extract in a small bowl until smooth. Gradually whisk in milk as needed to create a smooth and thick but pourable glaze.
Drizzle cooled biscotti with glaze. Let biscotti stand until glaze hardens, about 15 minutes.