Preheat the oven to 350°. Grease a 9-inch cake pan with nonstick spray, then dust with flour to coat.
In a medium pot, bring the butter and stout to a simmer, stirring until the butter is melted. Whisk in the cocoa powder. Cool slightly.
In a large bowl, whisk the flour with the baking soda, salt and sugar.
In a medium bowl, whisk the eggs with the milk and vanilla. Gradually add the butter mixture to the egg mixture, whisking well to combine.
Gradually add the wet ingredients to the dry ingredients, whisking until the batter is well combined and totally smooth. Gently fold in the chocolate chunks.
Pour the batter into the prepared cake pan. Bake until a toothpick or knife inserted into the center comes out clean (or with just a few moist crumbs), 60 to 75 minutes.
Let the cake cool for 30 minutes in the pan, then run a knife or spatula around the edge to loosen the cake from the pan; gently invert the cake onto a wire rack. Cool completely.
Make the whipped cream: Whip the cream to soft peaks in the bowl of an electric mixer fitted with the whisk attachment. Add the sugar and vanilla, and continue to whip until combined.
To serve, mound the whipped cream on top of the cake. The finished cake (before adding whipped cream) will keep for up to a week in an airtight container. With whipped cream topping, it will keep in the refrigerator for up to two days.