For the ice cream, in a saucepan, whisk the malted milk powder with a splash of milk until dissolved, then add the remaining milk and the cream and bring the mixture to a simmer.
In a bowl, whisk the egg yolks and caster sugar until pale and fluffy, then pour the warm milk mixture over the egg mixture, whisking continuously until well combined.
Return the mixture to the saucepan and cook over a gentle heat, whisking all the time, until the mixture thickens just enough to coat the back of a spoon.
If using a Pacojet machine, fill (but do not overfill) the container, then place it in the freezer for at least 4 hours, or until frozen solid. Alternatively, pour the mixture into an ice cream machine and churn until frozen, then transfer the ice cream to an airtight, freezeable container and freeze until needed.
Meanwhile, for the doughnut, mix the flour and sugar in a large mixing bowl, or in the bowl of a free-standing mixer.
Heat 100g/3½oz of the butter and half of the milk in a saucepan over a low heat. When the butter is just melting, remove the pan from the heat and whisk in the yeast until dissolved. Set aside.
Add the remaining milk to the pan containing the yeast mixture, then pour all of the liquid over the dry ingredients. Mix for 5 minutes in the free-standing mixer or using an electric whisk fitted with a bread hook attachment, on a medium speed, until the mixture has come together as a soft, sticky dough. Alternatively, knead by hand on a lightly floured work surface for 5 minutes.
Place the dough into a large bowl and cover with cling film, then set aside at room temperature to prove for 1 hour, or until well risen, spongey and nearly doubled in size.
Grease the insides of a 38cm/15in deep-sided, non-stick cake tin with butter, and grease the outside edge of a 12.5cm/5in cake tin or ovenproof ramekin. Place one inside the other.
Melt the remaining 150g/5oz of butter and pour into a large, high-sided roasting tray.
Mix together the demerara sugar and cinnamon in another high-sided roasting tray.
Turn out the proved dough onto a lightly floured work surface. Knead it gently for a couple of minutes until smooth and elastic, then roll it into a big bun shape. Using a pair of scissors, snip the dough in the centre to create a hole reaching all the way from the top to the bottom of the bun; use your fingers to gently increase the size of the hole.
Dip the monster doughnut into the melted butter until completely coated, then dust it on all sides with the cinnamon sugar. Transfer the doughnut to the prepared tin, easing the doughnut hole around the smaller cake tin or ramekin. Cover the doughnut loosely with cling film and set aside at room temperature for 40-45 minutes, or until the dough is well risen.
Preheat the oven to 180C/160C Fan/Gas 4.
When the monster doughnut has proved, remove the cling film and bake in the oven for 25-30 minutes, or until golden-brown on top and cooked through. Carefully remove the inner tin, then turn the whole doughnut out onto a large serving platter.
Stir the icing sugar and bourbon together in a bowl until smooth, then spoon the mixture into a small piping bag and drizzle it over the top of doughnut.
Fill a ramekin small enough to fit inside the doughnut hole with the ice cream. Serve immediately.