To make the rum cake:
1 cup (230 grams) unsalted butter softened to room temperature
2 cups (400 grams) granulated sugar
4 large eggs room temperature
1 large egg yolk room temperature
2 teaspoons pure vanilla extract
2 and ¾ cups (305 grams) cake flour
2 and ½ teaspoons baking powder
½ teaspoon salt
¾ cup (180 ml) buttermilk
⅓ cup (80 ml) dark rum
1 cup (120 grams) chopped walnuts or pecans (optional)
To make the rum syrup:
½ cup (115 grams) unsalted butter
1 cup (200 grams) granulated sugar
½ cup (120 ml) dark rum
¼ cup (60 ml) water
1 teaspoon pure vanilla extract
To make the rum cake:
Preheat oven to 350°F (177°C).
In a large bowl, using a hand mixer, cream the butter and granulated sugar together until light and fluffy (about 4-5 minutes). Beat in eggs one at a time, then beat in the egg yolk and pure vanilla extract, stopping to scrape down the sides of the bowl as needed.
In a separate mixing bowl whisk together the cake flour, baking powder, and salt. Set aside.
In a large measuring cup whisk together the buttermilk and rum. Set aside.
Alternate mixing the dry ingredients with the buttermilk rum mixture into the wet ingredients in three additions, starting and ending with the dry ingredients.
Generously spray a 10-inch (12-cup) bundt pan with nonstick cooking spray, or melted butter, making sure to fully coat the entire pan. Sprinkle the chopped nuts into the pan and spread them around in an even layer. Scoop the batter into the prepared bundt pan over the chopped nuts and spread it around into one even layer.
Bake at 350°F (177°C) for 40-55 minutes or until a toothpick inserted into the cake comes out clean. Cover loosely with foil for the last 5-10 minutes of baking time if needed to prevent excess browning.
Remove the cake from the oven and place it on a wire rack to cool slightly in the pan while you prepare the glaze.
To make the rum glaze:
Slice the butter into pieces and place it in a large saucepan over medium heat. Once the butter has melted, add the sugar, rum, and water to the saucepan and stir until well combined.
Bring to a rapid boil over medium heat, stirring occasionally. Once the mixture begins to rapidly boil, reduce the heat to medium-low heat and simmer for 6-8 minutes, stirring occasionally. Remove from the heat, then stir in the vanilla extract.
Use a skewer or a fork, to poke holes in the cake, spoon about 1/3 of the glaze over the cake in the pan. Allow it to sit for about 10 minutes.
Place some foil or parchment paper under the wire rack, then carefully invert the cake onto the wire rack and remove it from the pan. Use a skewer to poke holes on the top of the cake and then carefully spoon the rest of the glaze on top of the cake and around the sides.
Allow the cake to cool completely, then serve, and enjoy!