For the crumble mix butter in small pieces with flour, sugar and cinnamon.
For the dough whisk soft butter, sugar and salt until creamy.
Add eggs one by one whilst mixing.
Mix in flour, baking powder and egg liqueur.
Spread dough evenly on baking tray with baking paper.
Half and pit plums and place on dough.
Top with crumble.
Bake in preheated oven at 180° C for about 50 minutes.