Butter a 6-cup baking dish or a 7-inch springform pan. Line the bottom of the pan with parchment paper or foil and butter the paper.

Break the chocolate into a heatproof bowl. Add the butter. Place the bowl over a pan of simmering water. The water should not touch the bottom of the bowl. When the chocolate is softened, remove the bowl from the heat and stir until blended and smooth. Stir in the rum or coffee.

In a medium bowl, beat the eggs with the sugar until light and pale yellow. Stir the egg mixture into the chocolate. Scrape the batter into the prepared pan.

Place the pan in the slow cooker. Cover and cook on high for 2 hours, or until set.

Remove the pan from the slow cooker. Cover and chill for several hours or overnight. To serve, run a small knife around the edge of the cake. Invert it onto a serving plate. Place the cocoa powder in a small strainer and sprinkle it over the cake.