Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4 and grease two 6-hole donut tins with the butter
Make the crumble first by putting all the ingedients into a bowl and rubbing the butter into the flour, sugar and cinnamon with your fingers until it resembles breadcrumbs. Set it aside
Make the donut batter by mixing together the caster sugar, milk, vegetable oil and eggs in a bowl with a whisk
Peel, core and chop up the apples into small pieces. Toss them in a little of the flour then stir them into the batter mix
Add the rest of the self raising flour and whisk in
Divide the batter between the trays. You can put the batter into a piping bag or a jug to make it a bit neater to dispense into the tins, but you can do it with a spoon too
Sprinkle the crumble evenly over the batter
Bake them for 15 minutes, then turn the oven up to 180C Fan/200C/375F/Gas Mark 5 for 5 minutes to brown the tops. Then remove them from the tin (I use a spoon to gently prise them out) and leave to cool on a cooling rack
Stir the icing sugar and cinnamon together in a bowl and add enough water to make a drizzling icing
Set the donuts onto a rack with a baking tray underneath to catch any drips, or set them on some baking paper
Either put the icing in a piping bag and drizzle it over the donuts or use a spoon to drizzle it
Store leftovers in an airtight container and eat within 3 days