Heat the oven to 325°F / 165°C. Grease a baking dish with butter and melt the remaining butter, then set it aside to cool.
Using a stand mixer or hand mixer on medium-high, whip the eggs and sugar together until the mixture becomes very thick and pale yellow, about 5-10 minutes.
Sift the cocoa powder and flour together in a separate bowl. Fold the dry ingredients into the egg mixture on low speed. Scrape in the vanilla bean seeds and add the framboise if using. With the mixer still on low, slowly stream in the cooled melted butter and mix until just combined and smooth.
Pour the batter into the prepared dish. Place the dish inside a larger roasting pan and fill the outer pan with hot tap water until it reaches about halfway up the sides of the baking dish.
Bake for 50-60 minutes. The edges should be set — a toothpick inserted about 2 inches from the edge should come out mostly clean — while the center will still appear slightly underdone.
Let it cool briefly, then serve warm with a scoop of vanilla ice cream.