Cut the banana and butter into smaller pieces. Put the banana, butter, vanilla sugar, flour, and oats into a mixing bowl, and stir everything together. Stir in the milk a little at a time until the dough comes together. The dough will be slightly sticky. Refrigerate the dough for 30 minutes. Turn the dough out onto a lightly floured surface and roll it out to a thickness of 1-2 millimeters. Cut the dough into biscuits in the desired size. If you don't have a cutter, you can use a glass instead. Place the biscuits on two baking trays lined with parchment paper and bake them one tray at a time. Bake the biscuits at 200 degrees Celsius (392 degrees Fahrenheit) (fan) for 7-10 minutes. Let the biscuits cool on a wire rack. Store the biscuits in a cookie tin with parchment paper at the bottom. The biscuits can be kept in a cookie tin for 10 - 14 days.