01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - Whisk together flour, baking powder, and salt in a medium bowl until well blended.
03 - Beat butter and sugar in a large bowl until light and fluffy, approximately 3–4 minutes.
04 - Add eggs one at a time, beating thoroughly after each addition. Mix in vanilla extract.
05 - Add flour mixture to butter mixture in three parts, alternating with grape soda. Begin and end with dry ingredients. Mix until just combined.
06 - Stir in grape extract and a few drops of purple food coloring until desired color is achieved.
07 - Divide batter evenly among cupcake liners, filling each about 2/3 full.
08 - Bake for 16–18 minutes, or until a toothpick inserted in the center comes out clean. Cool in pan for 5 minutes, then transfer to a wire rack to cool completely.
09 - Beat butter in a large bowl until creamy. Gradually add powdered sugar, mixing well after each addition.
10 - Add 2 tbsp grape soda, grape extract, and a pinch of salt. Beat until light and fluffy. Add more grape soda, 1 tsp at a time, if needed for desired consistency.
11 - Mix in purple food coloring until desired shade is reached.
12 - Frost cooled cupcakes with grape buttercream. Decorate with grape candies or sprinkles if desired.