Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan, or line with parchment paper leaving an overhang on the sides.
Step 2: In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt. Set aside.
Step 3: In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth.
Step 4: Add the eggs, one at a time, whisking well after each addition. Stir in 1/2 cup milk and 1 teaspoon vanilla extract.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
Step 6: Transfer about two-thirds of the chocolate batter to the prepared loaf pan and spread evenly.
Step 7: To the remaining one-third of the batter, add 1 teaspoon of pure peppermint extract and mix gently until just combined.
Step 8: Spoon dollops of the peppermint batter over the chocolate batter in the loaf pan. Use a knife or skewer to gently swirl the peppermint into the chocolate batter for a marbled effect. Be careful not to overmix the swirl.
Step 9: Bake for 50-60 minutes, or until a wooden skewer inserted into the center comes out with moist crumbs, but no wet batter. If the top browns too quickly, tent loosely with foil.
Step 10: Let the bread cool in the pan for 10-15 minutes before transferring it to a wire rack to cool completely.
Step 11: While the bread cools, prepare the glaze: In a small bowl, whisk together the powdered sugar, the remaining 1/2 teaspoon peppermint extract, and 2-3 tablespoons of milk until smooth and pourable. Add more milk if needed for desired consistency.
Step 12: Once the bread is completely cool, drizzle the peppermint glaze over the top. Immediately sprinkle with crushed candy canes, if using. Slice and serve!