Preheat the oven to 175°C (350°F). Grease an 8–9 inch (20–23 cm) springform or baking pan with softened Unsalted butter and sprinkle the bottom and sides with Fine dry breadcrumbs to prevent sticking.
In a small saucepan or microwave-safe bowl, melt about half of the butter until just melted; remove from heat and whisk in the Granulated sugar and Unsweetened cocoa powder until smooth and glossy.
Allow the chocolate mixture to cool slightly, then beat in the Large eggs one at a time until fully incorporated. Stir in the Vanilla extract and a pinch of Fine salt.
Gently fold in the All-purpose flour until just combined — the batter should be thick but still pourable; avoid overmixing to keep the cake tender and gooey.
Pour the batter into the prepared pan and smooth the top with a spatula.
Bake for 18–25 minutes at 175°C (350°F). The edges should be set and slightly craggy while the center remains soft and a bit jiggly — a toothpick inserted in the center should come out with moist crumbs, not clean.
Remove the cake from the oven and let it cool in the pan to room temperature so the center sets up a bit; the cake will be very sticky and fudgy when fully cooled.
Just before serving, dust the top with Powdered sugar for a pretty finish. For an optional accompaniment, whip Heavy cream with a little Powdered sugar to soft peaks and serve alongside slices of the kladdkaka.
