Make the cake. Preheat the oven to 350 degrees F. Grease and flour 3 6-inch baking pans that are 3 inches deep. Set aside.
In a medium bowl, mix together the flour, baking powder, spices, and salt together and set aside.
In a mixer, add the butter and sugar together and beat until light in color and fluffy. Add the vanilla and then beat in the eggs one at a time until well combined.
Alternate adding the flour mixture and the milk to the mixer. Begin and end with the flour, which should be added in three additions, while the milk is added in two additions.
Pour the batter into the baking pans and smooth out the tops. Bake for 25 – 30 minutes, or until a toothpick inserted in the middle comes out clean.
Allow to cool in the baking pans for 10 minutes, then run a knife between the edges of the pan and the cake and then turn out the cake onto a wire rack to cool to room temperature. Once cooled, tightly wrap in plastic wrap and freeze until needed.
Make the filling. Pit and dice the plums. Add all the ingredients into a small saucepan and heat on medium high until the liquid is thick. Cool completely.
Make the frosting, Beat the butter until it is smooth and creamy. Add the powdered sugar one cup at a time and beat well. Add in the plum syrup, vanilla, and salt together until well-combined. Add in the heavy cream one tablespoon at a time until you reach the consistency you desire.
Make the drizzle. Add the white chocolate and heavy cream to a microwave-safe bowl. Heat for 30 seconds on 50% power in the microwave, then mix. Repeat until the white chocolate is melted. Stir in the coconut oil. Allow to cool completely.
Assemble the cake. Cut off the domed tops of the cakes. Create a ring of frosting around the edges of two of the cake layers. Fill with the plum filling. Top these layers on top of one another, then top with the plain cake layer. Frost the cake with the remaining frosting, then chill in the refrigerator for at least one hour. Once chilled, pour the drizzle over the top of the cake and gently push the drizzle over the edges and allow it to drizzle down.