Preheat oven to 325°F and grease a 9-inch springform pan.
In a small saucepan, melt the butter. Once melted, remove from heat and whisk in the chocolate until smooth. Set aside to cool.
In a large bowl, whisk together eggs and sugar until foamy. Check that chocolate is cooled, then whisk into the egg mixture.
Gently fold in the flour and baking powder until incorporated.
Pour batter into prepared pan and bake for 25-30 minutes. Do not overbake; it should be set and look dry on top, but the interior should be gooey (not runny).
Let cool completely before serving. Dollop with whipped cream and tumble berries on top, or top each slice individually.
Notes
If you don't have caster sugar, blitz granulated sugar in a food processor to make the superfine sugar.
If using an 8-inch springform pan, make sure to increase the baking time to ensure to center is not oozing.