Substitute extra cream for rum in the sauce, if you like. For a thicker glaze, add the whole 1 1/3 cups powdered sugar, whisking constantly, as the glaze begins to cool in the pan. For a sheer, saucy glaze, use less powdered sugar.
2 cups butter, softened
3 cups firmly packed light brown sugar
6 large eggs
3 cups all-purpose flour
2 teaspoons baking powder
2 teaspoons ground cinnamon
2 teaspoons ground ginger
1/2 teaspoon ground allspice
1/4 teaspoon baking soda
1 cup buttermilk
2 cups toasted chopped pecans
1 tablespoon vanilla extract
1/4 cup firmly packed light brown sugar
1/4 cup granulated sugar
1/4 cup butter
1/4 cup heavy cream
1 tablespoon rum
1/2 to 1 1/3 cups powdered sugar
How to Make It
Prepare Cake: Preheat oven to 325°. Beat 2 cups butter at medium speed with a heavy-duty electric stand mixer until creamy. Gradually add 3 cups brown sugar, beating until light and fluffy. Add eggs, 1 at a time; beat until blended after each addition.
Stir together flour and next 5 ingredients. Add flour mixture to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed just until blended. Stir in pecans and vanilla. Spoon batter into a greased (with shortening) and floured 15-cup Bundt pan.
Bake at 325° for 1 hour and 15 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack 15 minutes; remove from pan to wire rack, and cool completely.
Prepare Glaze: Bring 1/4 cup brown sugar and next 3 ingredients to a boil in a small saucepan over medium heat, whisking constantly; boil, whisking constantly, 1 minute. Remove from heat, and whisk in rum. Add 1/3 cup powdered sugar; whisk until smooth. Gradually add more powdered sugar, whisking constantly, until desired thickness is reached. Drizzle glaze over cake.