Add butter to a bowl of a stand mixer or a handheld mixer with the whisk attachment and mix for about 10 minutes until it is pale in colour and and looks like whipped butter. If the butter is not soften enough these may take about 15-20 minutes.Don't rush this step, It is very important to do this step.
Sift the icing sugar, cocoa powder directly onto the butter
Mix it well for 3 minutes at high speed until it is thick and creamy
Add in the vanilla essence, Milk and beat it for about 10 minutes on a low setting until fluffy and creamy