Preheat oven to 350 degrees (F).
Cream butter and sugar together in a mixing bowl. Add egg and buttermilk.
Slowly incorporate flour, baking powder, salt, and cinnamon.
Once mixture is smooth, fold in berries, careful not to damage the berries.
Pour into a greased 8 x 8-inch pan.
In a separate bowl, fork together butter, oats, brown sugar, and spices.
Crumble over the top of the batter and bake for 30 - 45 minutes, depending on the juiciness of the berries. A toothpick inserted into the center of the cake should come out mostly clean. A few moist crumbs clinging to it is okay.