In a mixing bowl, beat together the following ingredients until well-blended and smooth: softened butter, sugar, eggs, vanilla.
In a separate bowl, use a stand mixer to combine the flour, baking powder, salt and half and half. I recommend adding the flour a little bit at a time, alternating the flour with half and half, until all have been added and mixing between additions.
Scrape the sides of the bowl and mix on high speed for one minute.
Remove mixing bowl from the mixer. Scoop about a third of the batter into a smaller bowl and add approximately 1/4 tsp of the purple food coloring gel to the batter. Swirl the gel through the batter with a spoon. Leave a swirl design in the batter (do not mix it together completely).
Place cupcake liners in a 24 count muffin tin or two 12 count tins.
Fill each tin/liner about halfway with the plain white batter. Then add a small amount of the swirled batter on top.
Each liner should be filled about 3/4 of the way.
Bake at 350 degrees for 18 to 20 minutes, or until a toothpick inserted in the center comes out clean. When the cupcakes are done, remove from the oven to a wire rack, and let them cool completely.