Preheat the oven to 190C/375F. Cut the plums in half and remove stones. In a large bowl combine the pitted plums, brown sugar, flour and the blackcurrant liqueur if using. Set aside.
In another bowl combine the flour, baking powder, sugar and salt. Then add the cold, cubed butter and with a pastry blender cut the butter into the flour mixture until crumbly. Then add the oats and chopped walnuts and stir briefly. Add the milk and stir until just moistened.
Add the plum and sugar mixture to the bottom of a deep 9 inch pie plate or a 2 Quart/2 Litre baking dish. Drop spoonfuls of the cobbler topping mixture on top of the sugared plums leaving space in between. Bake in the preheated oven for 40-45 minutes until the top is golden.