Combine butter and 1.6 litres of water in a large heavy-based saucepan and bring to the simmer over medium-high heat. Gradually rain in polenta, whisking continuously, then reduce heat to low and cook, stirring continuously, until thick and smooth and no longer grainy (30-45 minutes). Add parmesan (reserving 1 tbsp), season generously with sea salt and freshly ground black pepper and pour into an oiled 25cm-square cake pan. If polenta doesn’t have a silky pouring consistency, add 100ml boiling water to thin. Cool, refrigerate until firm (2-3 hours) then, using a 9.5cm-diameter pastry cutter, cut into rounds.
Meanwhile, for Gorgonzola sauce, combine ingredients in a saucepan, bring to the simmer over low heat, remove from heat and gently whisk to combine. Refrigerate until completely cold (1-1½ hours).
Preheat oven to 230C. Place a polenta round in the centres of six buttered, heatproof shallow bowls. Spoon sauce onto polenta without spilling, scatter over remaining parmesan and bake on bottom shelf until golden (12-15 minutes). Serve immediately.