Preheat oven to 350°F. Grease a 9-inch x 5-inch loaf pan and line with a parchment paper sling.
Whisk together flour and baking powder in a medium bowl.
Place butter, sugar, and salt in the bowl of a stand mixer fitted with a paddle attachment. Cream until very light and fluffy, 3-5 minutes.
Beat in the eggs, one at a time, scraping down the bowl as needed. Mix in vanilla.
Add half the dry ingredients and mix until just combined. Beat in the milk until well mixed, then add the remaining dry ingredients until just combined.
Transfer 1 ½ cups of batter into a separate bowl and stir in remaining 1 tablespoon milk, cocoa powder, peppermint extract and a few drops of red food coloring.
Spoon the two batters alternatively into prepared pan, following a checkerboard pattern until all of the batter has been used. Use a thin knife or skewer to swirl an “S” shape through the batter 2 to 3 times. Tap the pan against the counter to flatten the batter and bake for 55-65 minutes, or until a toothpick inserted into the center comes out clean. If the top is browning too quickly, cover the top loosely with aluminum foil.
Let cool 10 minutes, then carefully lift the parchment paper sling out of the pan onto a rack to cool completely.
Spoon vanilla glaze (recipe below) over the top of the cake and let it drip over the sides. Sprinkle with crushed peppermint candies and let sit 15 minutes to set. Slice and serve.
Combine powdered sugar and vanilla bean paste in a medium bowl. Add heavy cream and whisk until combined and smooth. Glaze should be thick but pourable; add more cream, a teaspoon at a time, until desired consistency is reached.