Preheat the oven to 325°F and brush the inside of a 12” Bundt pan completely with butter. Dust flour into the pan, rotating to get every nook coated. Add more flour as needed, shaking any excess out into the sink. (This helps ensure your cake doesn’t get stuck.)
Combine the butter and sugar in the bowl of a stand mixer fit with the paddle attachment. Beat on medium-high for about 7 minutes, until light and fluffy. Scrape down the sides of the bowl and add the eggs one at a time, mixing completely between each addition. Add the vanilla and lemon zest.
With the mixer on low, add 1 cup cake flour and salt, followed by half of the 7UP® Shirley Temple. Add 1 more cup flour, then mix in the remaining 7UP® Shirley Temple. Stir in the last cup of flour, mixing just until no streaks remain.
Fold in a few drops of food coloring if desired, mixing just until combined.
Pour the batter into the prepared pan and bake for 1 hour to 1 hour 15 minutes, until a toothpick inserted comes out clean.
Remove the pan from the oven and let sit for 1 minute. Cover the Bundt with a wire rack or cutting board and use a tea towel to flip it over, knocking on the top to release the cake from the pan. Carefully lift the pan off the cake and let cool.
Decorate with your favorite icing before serving. (Optional)
To make the icing
Add the butter to the bowl of a stand mixer fitted with the whisk attachment and beat on high for 1 minute. With the mixer on low, add the vanilla and mix to combine. Scrape down the sides of the bowl.
Add the salt and mix, then add the sugar one cup at a time, until completely incorporated. Scrape the sides of the bowl again and mix on high for 3 minutes, until light and fluffy.
Stir in the milk 1 tablespoon at a time, until the icing is smooth and
Add a few drops of food coloring, mixing until no streaks remain. (Optional)
Transfer the icing to a piping bag and pipe over the cooled Bundt cake. Scatter rainbow sprinkles on top and serve.
