Preheat the oven to 350˚F. Butter and flour six 6-ounce ramekins or one 9-inch springform pan lined with parchment paper.
To make the topping: In a medium bowl, stir together the butter, flour, sugar, salt, cinnamon, and nutmeg. The mixture should be crumbly in texture. Set aside.
To make the base: In a medium bowl, whisk the flour, salt, and baking powder. Set aside. In the bowl of a stand mixer, beat the butter and sugar on medium speed until pale and fluffy, about 1 minute. Add the vanilla and egg and beat until smooth. Scrape the sides of the bowl. Add the flour mixture and milk and mix for 1 minute. Using a rubber spatula, fold the blackberries into the batter. Pour the batter into the prepared ramekins or springform pan and sprinkle with the topping. Bake until a toothpick inserted into the center of the cake comes out clean, about 1 hour. Let cool before serving.