Preheat the oven to 350°F (180°C). In a medium bowl combine the flour, cocoa, baking powder and salt. Set aside.
Melt the butter and 3.5 oz (100g) semisweet chocolate together over low heat over a bain-marie. Remove from heat and let cool slightly.
Combine the eggs and sugar and beat at medium speed with a hand mixer until light and fluffy, about 3-5 minutes. Add the chocolate mixture and stir until well combined. Add rum and rum extract.
Add the flour mixture and stir until just combined. Fold in the chocolate chips or chunks.
Drop 1 tablespoon portions 2 inches (5cm) apart onto a baking sheet lined with parchment paper.
Sprinkle sea salt flakes on top of each.
Bake for 9–10 minutes, the cookies should be slightly underbaked for a chewy, soft interior.
While they’re still warm, press a few extra chocolate chips on top for a beautiful, finished look. Let the cookies cool for 5 minutes on the baking sheet.
Cool completely on a wire rack.