This recipe is based off of the one my grandfather would make for his suet pudding (sounds gross but it was so delicious and decadent). He always used brandy, but I like it with vanilla sometimes too.
1 stick (8 tablespoons) unsalted butter, softened
1 1/2 cups confectioner's sugar
2 tablespoons vanilla extract or brandy, or a mix of the two
Cream butter in a mixer until just a little fluffy. Add in sugar and beat until smooth (scrape the sides of the bowl to make sure everything gets fully incorporated). Once it's all mixed together add in the vanilla and/or brandy and let the mixer go until it's completely smooth and well mixed.
Refrigerate until ready to use, let soften slightly at room temperature before serving.