Heat oven to 300 degrees. Brush a 9-by-4 1/2-inch loaf pan with butter. Line pan with parchment; brush
with butter. Set aside.
Combine the prunes, raisins, and currants in a medium bowl. Add 1/2 cup stout, and let stand.
Sift flour, baking powder, nutmeg, and cinnamon. In the bowl of an electric mixer fitted with the paddle attachment, cream butter and sugar until fluffy, about 3 minutes. Add eggs, one at a time, mixing
well after each, scraping down sides twice. Add dry ingredients in two additions; mix just to combine. Fold in fruit mixture.
Pour batter into pan. Bake until dark brown and set and a cake tester inserted into the middle of cake comes out clean, about 3 1/2 hours. (Cracks will appear on top of cake.) Remove from oven; sprinkle with 1/2 cup stout. Let stand on wire rack 30 minutes. Remove from pan; discard parchment; let cool completely.
Wrap in cheesecloth or muslin. Douse fruitcake with remaining 1/4 cup stout. Store in a cool, dark, dry place, dousing with 1/4 cup stout once a week for at least 1 month before serving.