Grease and line three 20cm cake tins. If they’re loose-bottomed, place them onto a couple of baking trays to catch any batter which drips through. Preheat the oven to 180 C (350 F).
In a small pot, melt the butter over a low heat until just melted. Pour into a large bowl and mix together with the oil, cocoa powder, malted milk powder, sugar, eggs and yogurt until well combined. Pour in the just-boiled water and quickly (but carefully) mix together until smooth.
Add in the flour, baking powder, bicarbonate of soda and salt. Use a whisk to gently stir together until the flour has just been mixed in.
Divide the batter between lined tins and bake for 35-45 minutes until toothpick inserted into the centre comes out mostly clean – ok if a few crumbs are stuck – and when gently poked, the cake springs back. Leave to cool for a few minutes then gently loosen the edges of the cake with a knife. Leave to cool completely then pop out of the tins.
Once cooled, level the cakes using a serrated knife if the layers have domed tops. Use the frosting to sandwich the layers together and to coat the entire cake. Decorate with crushed maltesers.