Step 1: Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners.
Step 2: In a medium bowl, whisk together 1½ cups all-purpose flour, 1½ teaspoons baking powder, and ¼ teaspoon salt. Set aside.
Step 3: In a large bowl, cream together ½ cup (1 stick) unsalted butter, softened, and 1 cup granulated sugar using an electric mixer until light and fluffy, about 2-3 minutes.
Step 4: Beat in 2 large eggs, one at a time, mixing well after each addition. Then, stir in 2 teaspoons pure vanilla extract.
Step 5: Gradually add the dry ingredients to the wet ingredients, alternating with ½ cup whole milk. Begin and end with the dry ingredients, mixing until just combined. Be careful not to overmix.
Step 6: Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full.
Step 7: Bake for 18-22 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cupcakes cool in the muffin tin for a few minutes before transferring them to a wire rack to cool completely.
Step 8: For the vanilla buttercream, in a large bowl, beat 1 cup (2 sticks) unsalted butter, softened, until creamy. Gradually add 3 cups powdered sugar, one cup at a time, beating well after each addition until smooth. Mix in 1½ teaspoons pure vanilla extract and 1-2 tablespoons heavy cream or milk until the frosting is light, fluffy, and spreadable. Adjust liquid as needed.
Step 9: For the chocolate buttercream, transfer about 1½ cups of the prepared vanilla buttercream to a separate bowl. Sift in ¼ cup unsweetened cocoa powder and add ½ teaspoon pure vanilla extract. Beat until smooth and well combined. If it seems too stiff, add a tiny splash of milk or cream.
Step 10: Prepare your piping bag with a large star tip. Lay out a long piece of plastic wrap. Spoon the vanilla buttercream in a line down one half of the plastic wrap, and the chocolate buttercream in a line down the other half, side-by-side. Carefully roll the plastic wrap into a log, twisting the ends to seal. Snip one end of the plastic wrap log and insert it into your piping bag.
Step 11: Frost the cooled cupcakes with the chocolate swirl buttercream, starting from the outside edge and swirling towards the center. Garnish with chocolate shavings or sprinkles, if desired. Serve and enjoy!