01 - Preheat oven to 350°F. Grease and lightly flour a 9-inch round cake pan or line it with parchment paper.
02 - Whisk together flour, baking powder, and salt in a medium bowl; set aside.
03 - Beat softened butter and granulated sugar in a large bowl until light and fluffy.
04 - Add eggs one at a time, beating well after each addition; then mix in vanilla and peppermint extracts.
05 - Gradually add flour mixture and milk alternately to the butter mixture, starting and ending with flour; mix until just combined.
06 - Divide batter evenly; tint one portion with red gel food coloring until vibrant.
07 - Spoon alternating dollops of plain and red batter into the pan. Use a skewer or knife to gently swirl together, avoiding overmixing.
08 - Bake for 30 to 35 minutes, until a toothpick inserted in the center comes out clean. Cool in pan 10 minutes, then transfer to a wire rack to cool completely.
09 - Beat butter until creamy. Gradually add powdered sugar, milk, peppermint extract, and salt. Beat until smooth and fluffy.
10 - Tint a small portion of frosting with red food coloring and fold gently for a swirl effect before spreading.
11 - Spread frosting evenly over cooled cake. Decorate with crushed peppermint candies if desired. Slice and serve.