Place the diced candied fruits in a bowl and pour in the rum. Stir gently with a spatula to coat the fruits evenly. Let them macerate for at least 30 minutes to soak up the rum's flavor.
In a large mixing bowl, combine the softened butter and granulated sugar. Beat the mixture until it becomes smooth and creamy.
Gradually add the room-temperature eggs, one at a time, into the butter and sugar mixture and continue mixing.
Pour the rum-soaked candied fruits (including any remaining rum) into the batter. Mix gently to distribute the fruits evenly.
Add the all-purpose flour and baking powder to the bowl. Continue mixing until the batter is smooth, without any lumps.
Transfer the batter to a greased loaf pan, smoothing the surface with a spatula.
Preheat your oven to 350°F (180°C). Bake the cake for 20 minutes, then reduce the oven temperature to 340°F (170°C) and bake for an additional 20 minutes.
Insert a knife into the center of the cake. If it comes out clean, the cake is done. If the blade comes out with wet batter, return the cake to the oven for an additional 5–10 minutes, checking frequently.
Once baked, remove the cake from the oven and let it cool slightly in the pan before turning it out onto a wire rack. Allow it to cool completely before slicing. Bon appétit!