Beat butter in an electric mixer until light and very fluffy (6-8 minutes). Add icing sugar in batches, beating well to combine (10-15 minutes), then add spices and vanilla seeds and beat to combine well (4-5 minutes), then, with motor running, drizzle in brandy, beating well to incorporate. Beat in eggwhite until light and very fluffy (4-5 minutes). Transfer to sterilised jars and refrigerate until required. Bring to room temperature before using. Whipped brandy butter will keep refrigerated for up to 2 weeks.