How to Make It
Preheat oven to 350°F. Grease and flour a 15-cup Bundt pan. Combine raisins and ¾ cup of the rum in a small bowl and microwave for 30 seconds. Drain raisins; reserve rum.
Whisk baking powder, salt, and 3½ cups of the flour in a medium bowl. Toss together drained raisins, grated apple, and remaining ½ cup flour in a small bowl.
Beat butter, dark brown sugar, and granulated sugar in a large bowl with an electric mixer on medium until light and fluffy, about 2 minutes. Add eggs, 1 at a time, beating after each addition; beat in vanilla.
Add flour mixture in 3 parts and reserved rum in 2 parts, beginning and ending with flour mixture. Mix just until combined. Fold in raisin mixture.
Spoon batter into prepared Bundt pan and bake until a toothpick comes out clean, 65 to 75 minutes.
Meanwhile, whisk together cardamom, 1 cup of the powdered sugar, and remaining 6 tablespoons rum in a small bowl.
Pierce bottom of warm cake all over with a fork and drizzle with ½ cup of the cardamom glaze; let stand for 20 minutes. Invert cake onto a plate and let cool completely.
Whisk milk, remaining 1 cup powdered sugar, and remaining cardamom glaze. Drizzle over cake.