01 - Preheat oven to 350°F (175°C) and line a standard 12-cup muffin pan with cupcake liners.
02 - Whisk together all-purpose flour, baking powder, and salt in a medium mixing bowl.
03 - In a large mixing bowl, beat softened butter and granulated sugar until pale and fluffy, approximately 2 to 3 minutes using an electric mixer.
04 - Add eggs individually, integrating thoroughly after each addition. Mix in vanilla extract until combined.
05 - Alternately add the dry flour mixture and milk to the butter mixture, starting and ending with flour. Blend gently until just incorporated.
06 - Spoon the batter into the prepared muffin cups, filling each evenly. Bake for 16 to 18 minutes or until a toothpick inserted in the center emerges clean.
07 - Allow cupcakes to cool in the baking pan for 5 minutes, then transfer to a wire rack to cool completely.
08 - In a clean bowl, beat unsalted butter until creamy. Gradually incorporate powdered sugar, beating until smooth. Mix in watermelon juice, vanilla extract, and food coloring if desired. Whip until light and fluffy.
09 - Set aside a small portion of frosting and tint it with green food coloring for optional edge decoration.
10 - Frost fully cooled cupcakes with the watermelon frosting. Garnish each with mini chocolate chips to simulate watermelon seeds and, if desired, pipe a green border around the edge for a rind effect.