Pre-heat the oven to 160°C fan. Generously butter a large bundt tin or spray with cooking spray.
In a stand mixer (or with an electric whisk), whisk the eggs and caster sugar together on medium-high speed for 5–7 minutes, until very pale, thick, and tripled in volume.
Meanwhile, melt the butter, dark chocolate, milk chocolate, malt syrup, and milk together in a heatproof bowl set over a pan of gently simmering water (or in short bursts in the microwave). Stir until smooth.
With the mixer running on low, slowly pour the warm chocolate mixture into the whipped eggs. Mix until just combined.
Sift together the flour, cocoa powder, and salt, then gently fold into the batter with a spatula until no streaks remain.
Pour into the prepared bundt tin and smooth the surface. Chill until ready to bake.
Bake for 20 minutes, until the edges are just set but the centre still has a gentle wobble.
Leave to cool in the tin for 10 minutes, then carefully turn out onto a plate. Serve warm at the table with scoops of ice cream and plenty of cream.
