Place a saucepan under an extraction (exhaust) fan, pour in the Guinness and set over high heat.
Reduce to half the quantity (1 cup/250 milliliters). Set aside to cool.
Melt the butter in a separate pan, then leave it to cool.
Butter 2 jelly roll pans, each about 12 x 8 inches (30 x 20 centimeters), and line with parchment paper.
Preheat the oven to 350˚F (180˚C).
Place the flour, sugar, cocoa powder, baking soda, baking powder, and salt in a bowl, whisk to mix, then make a well in the center.
Add 1/2 cup (125 milliliters) of the Guinness reduction with the cooled melted butter, egg and buttermilk and whisk the batter until no lumps are left.
Divide the mixture between the 2 prepared pans, then bake for 10 minutes. Insert a metal skewer into the center: when it emerges, it should be clean; also the cake should bounce back to the touch. It should be just cooked, rather than overcooked. The mix is wet and you really need to use a metal skewer to check it's done in the center.
Leave in the pans to cool, then chill. The cake needs to be chilled when you're cutting it to assemble the cake.
Halve both chilled cakes widthways, then trim the cakes so you have 4 matching flat layers that will sit neatly on top of each other.
Whip together the cream, confectioners' sugar, buttermilk, mascarpone, and vanilla seeds to soft peaks.
Use this to sandwich the cake layers together.
Serve in slices, so you see the layers.