Preheat the oven to 180C & grease & line a 9 x 9-inch brownie tin with baking parchment; set aside.
In a large bowl, combine the flour with the brown sugar. Pour in the cooled brown butter & mix until a ball of dough forms. Neatly press the dough into the prepared tin until evenly dispersed & bake in the preheated oven for about 20 minutes, or until lightly golden. Set aside to cool.
Next, make the crisp topping. Using your fingertips, combine all of the ingredients in a large bowl until crumbs start to form. Set aside.
Now, the filling. Add the sugar, vanilla extract, & lemon juice to a large bowl & use your fingers to fully incorporate. Mix in the spices and the flour. Finally, add the plums & blackberries to the bowl & toss thoroughly.
Pour the filling over the par-baked crust & sprinkle the crisp topping evenly over the top. Bake in the oven for 30-40 minutes, or until the fruits start to release their juices.
Leave to cool completely before slicing. Enjoy slightly warm with some whipped cream!