Preheat the oven to 320F and prepare a large brownie pan with cooking spray and baking parchment.
Place a large, heatproof bowl over a saucepan of just simmering water and add the butter and chocolate. Allow to melt, making sure the bowl does not touch the water, stirring only occasionally until the chocolate is melted and the mixture is smooth. Remove from the heat to cool a little.
While the chocolate is cooling, using an electric hand whisk beat together the eggs and sugar until light, thick, pale and frothy.
Using a metal spoon, fold the chocolate mixture into the egg mixture. Stir in the vanilla, then fold in the flour and cocoa to make a batter.
To make the pumpkin swirl, again whisk together the sugar and eggs until light, thick, pale and frothy. Fold in the pumpkin puree, followed by the flour and the spices.
Pour the brownie batter into the prepared pan. Dollop over the pumpkin mixture, and gently swirl into the chocolate with a teaspoon.
Bake for 45 minutes and allow to cool on a wire rack before slicing.