With a pastry brush or your hands, grease the loaf pan. Add a little bit of flour and shake the pan until the whole pan is coated.
Allow the butter to come to room temperature. Cut it into chunks and add them to a large bowl.
With your handheld mixer, blend the butter until creamy and white.
Add the sugar and mix on the highest speed until creamy.
Add the eggs one by one. Blend on high until each egg is completely incorporated into the butter mixture.
Add the ground hazelnuts to the butter-egg mixture and blend to combine.
In a separate bowl, combine the flour, bread crumbs, baking powder, and sea salt and mix them well.
Add a bit of the flour mixture and blend. Then add some of the rum and mix again. Continue until you have used up all the dry ingredients. Add just enough rum to make the cake batter drop down heavily from a spoon.
Bake the cake in a preheated oven at 325˚F for about 90 minutes or until a toothpick comes out clean. Let the cake cool down on a wire rack for about 8-10 minutes or until it is cool enough to touch.
Place a dinner plate upside onto the cake and carefully flip the cake over. It should fall easily out of the loaf pan. If it sticks, return it back to upright and carefully run a small knife around the edges and try again.