For the Royal Icing Transfers:
In a clean mixing bowl with the paddle attachment mix water, cream of tartar, and meringue powder until foamy.
Add all the powdered sugar at one time, cover the bowl with a towel, and start mixing on a low speed until combined. Increase speed and mix until soft peaks form then add vanilla and mix until stiff peaks.
To thin royal icing to pipeable consistency add water 1/2 tsp at a time stirring thoroughly until a line drawn through the icing disappears after 20-25 seconds.
Color with your preferred colors. I used black, red, blue, green, yellow, orange, and purple.
Place a sheet of wax paper over your template, pipe hearts (I made 40 hearts of each color) and silhouette and allow to dry overnight.
Store leftover royal icing in an airtight container in the fridge. Allow to come to room temperature before using again.
For the Cake:
In the bowl of a stand mixer combine sugar, flour, baking powder, and salt. In a separate bowl whisk together vegetable oil, egg, vanilla, and milk.
With the mixer on low slowly add in milk mixture and mix until combined, scraping the sides of the bowl.
Divide batter evenly between 3 lined and prepared 6" cake pans and bake at 350 degrees for 22-25 minutes or until a toothpick inserted in the center comes out clean.
Allow cakes to cool in the pans for 5 minutes before turning them out onto a cooling rack to cool completely.
For the Frosting:
In a stand mixer cream room temperature butter until pale and fluffy.
Add powdered sugar 1/2 a cup at a time until completely combined. Add vanilla.
Add milk if needed to thin frosting to your preferred consistency.
Level each cake and set first layer on your cake board, cake plate, or turntable. Add 1/4 cup of frosting and smooth into an even layer and repeat with the second and third layers.
Crumb coat the cake and refrigerate for 15 minutes.
Frost the cake using the remaining frosting and while the icing is still soft decorate it with your royal icing transfers*.