Preheat the oven to 350F. Line a 7" round cake tin with parchment and lightly grease with butter. Set aside.

In a bowl of a stand mixer fitted with a paddle attachment, sift in the flour and baking powder. Mix to combine. Add the butter, eggs, sugar, and vanilla. Mix on medium high speed until thoroughly combined, 2 minutes. Scrape the batter into the prepared pan and smooth the top of the batter with an offset spatula.

Bake for 25 minutes or until golden brown and a skewer comes out clean when inserted in the center. Remove from the oven and let it cool for a 15 minutes before removing the cake from the tin.

Meanwhile, beat the softened cream for the filling in a bowl of a stand mixer fitted with a paddle attachment. Beat on medium speed until smooth and light in color.

Slowly, add the powdered sugar in 2 to 3 additions. Mix on low speed until well combined. Add the vanilla extract and mix to combine. Add the milk and beat again for another 3 minutes. The cream should be smooth and light. If you find it's very dense and heavy, add a bit more milk.

Slice the cake evenly in half. Place some of the buttercream in a piping bag fitted with a large round tip. Pipe the cream around the circumference of the bottom layer. Fill the center with jam and spread it out evenly. Pipe more cream on top of the jam to create an even layer.

Place the second layer of cake on top. Spread the buttercream around the cake in an even layer. Refrigerate to set.

Tint the rest of the cream with the purple gel food coloring. Put the purple buttercream in a piping bag fitted with a small piping tip.

Following the scene from the film, write, "Happy Birthday Luna" and draw a spaceship in the center. Smooth it out the cream with your offset spatula.