In a small saucepan, melt half of the butter. Continue to cook, swirling the pan occasionally, until it turns brown and smells nutty; the foam will turn golden, too. Pour into a large bowl and add the rest of the butter.

Put the brown sugar into the same saucepan along with the cream, put it over the heat and stir until it starts to bubble and the sugar has melted. Scrape it into the bowl with the butter, add half the icing sugar and beat until smooth.

Add the remaining icing sugar and beat until smooth; add a little extra sugar or cream if it needs it to achieve a spreadable consistency.