How to Make It
Make the Cake: Preheat oven to 325°F. Grease and line two 9-inch round cake pans with parchment paper; set aside. In the bowl of a stand mixer, stir together cake flour, sugar, baking powder, baking soda, and salt. Add butter and beat on low speed until completely dissolved into dry ingredients. In a small bowl, whisk together buttermilk, eggs, oil, and extracts. Add half the liquid ingredients to the dry and beat on medium for 3 minutes. Scrape down sides of the bowl, add remaining liquid ingredients, and beat on medium-high for 2 minutes. Divide batter between cake pans, spread evenly, and bake until the cake springs back to the touch, about 30 minutes. Allow to cool in pans completely before removing.
Make the Filling: Combine dates, raisins, water, honey, and 1 tsp. rum extract in a medium saucepan. Stir over medium heat until water is absorbed and fruit is syrupy. Remove from heat; add 1/2 tsp. rum extract and walnuts.
Make Frosting: With the whisk attachment in a stand mixer, beat egg whites and salt until stiff peaks form. In a saucepan, combine sugar, water, and corn syrup over medium-high; stir until sugar is dissolved, then stop stirring and allow to boil. Cook until a candy thermometer registers at 238°F. Continue whipping egg whites on medium speed as you slowly stream in the hot sugar mixture. Whip until frosting has cooled completely, 6-8 minutes. Add vanilla and mix to combine. Take 1 cup of frosting and fold in to fruit and nut mixture.
Assemble Cake: Cut each cake layer in half horizontally using a long serrated knife. On one cake layer, evenly spread 1/3 fruit and nut mixture. Place another layer on top and repeat process of layering with fruit and nut mixture. Add third layer and cover with remaining fruit and nut mixture. Place final layer on top and cover entire cake with Frosting, using a spatula or spoon to create decorative swirls.