1. Preheat oven to 175°C (350°F).

2. In a large bowl, cream butter and granulated sugar together until light and fluffy, about 3–4 minutes.

3. Add eggs one at a time, beating well after each addition, until fully incorporated.

4. In a separate bowl, whisk together flour, baking powder, baking soda, and salt.

5. With the mixer on low, add the dry ingredients to the butter mixture in three parts, alternating with the milk in two parts, starting and ending with dry ingredients. Mix until just combined.

6. Melt chocolate in a heatproof bowl over simmering water or in 30-second bursts in the microwave, stirring between each interval. Let cool slightly.

7. Fold the melted chocolate into the batter until uniform. Stir in the rum.

8. Pour batter into a greased and floured 25 cm (10-inch) round cake pan with high sides, or a 2.5 L (10.5 cup) pan. Smooth the top with a spatula.

9. Bake in the lower third of the oven for 50 to 60 minutes, or until a toothpick inserted in the center comes out clean.

10. Let cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely.

11. Dust lightly with powdered sugar just before serving.