Preheat oven to 350°F. Grease and flour an 8-inch round cake pan.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large bowl, beat the butter and granulated sugar together until light and fluffy, about 3 minutes.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Add the flour mixture to the butter mixture in three parts, alternating with the sour cream and beginning and ending with the flour mixture. Mix just until combined.
Fold in the chopped damson plums. Pour the batter into the prepared cake pan and smooth the top. Sprinkle the sliced almonds and demerara or turbinado sugar over the batter.
Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
Serve the damson cake warm or at room temperature, with a scoop of ice cream or a drizzle of whipped cream, if desired. Enjoy!