Preheat the oven to 350° F. Grease an 8-inch by 8-inch baking dish.
Blend the dry ingredients:
Whisk together the flour, baking powder, and salt in a medium bowl. Hold it aside.
Prepare the fruit:
Peel the apples and remove the cores. Slice the apples into small cubes around 1 to 1½ inches in size.
Slice any large blackberries in half so that your whole berries and berry pieces are all about 1 to 1½ inches in size.
Mix the wet ingredients:
Pour the granulated sugar, brown sugar, and melted butter into a large bowl, and stir them together into a smooth mixture.
Add in the eggs, stirring to fully incorporate them with the butter and sugar. Stir in the vanilla extract.
Combine the mixtures:
Pour about half of the flour mixture into the bowl with the butter-egg mixture. Fold the flour in using a few strokes.
Pour in half of the milk, and stir it in. Add in the rest of the flour mixture and stir. Pour in the last of the milk. Stir the batter until it looks smooth.
Fold in the diced apples and the blackberries. Don't overmix, but try to distribute the fruit as evenly as you can through the batter.
Spoon the batter into the prepared baking dish and even out the top.
Bake
Bake the cake for 50-55 minutes. The top of the cake should be golden brown with a few cracks, and a knife inserted in the center should come out clean.
Move the pan to a cooling rack and let the cake cool completely.
While the cake bakes, make the Vanilla Hard Sauce:
Put the 2 tablespoons of butter, vanilla, and confectioner's sugar into a small bowl.
Beat the ingredients together with a hand mixer or a spoon.
Add in the whiskey, and continue beating the mixture to make a smooth, spoonable sauce. If it's too thick you can add in a teaspoon or two of water - or more whiskey, but this will make it even boozier!
Serve:
When the cake is cool, cut it into slices. Serve the slices with a little drizzle or a generous spoonful of the whiskey hard sauce.
Option: Booze-free sauce
To make a nonalcoholic sauce, replace the whiskey in the sauce ingredients with milk.
Notes
I love using McIntosh apples in this recipe, which are both tart and sweet, and are a great choice for baked goods including pies.
No need to lug out your stand mixer for this recipe: it's a cinch to stir together by hand, which also lessens the chance of overmixing the batter.
If you prefer to skip the hard sauce, the apple blackberry cake is sweet and moist enough to serve all on its own. You can also make a nonalcoholic sauce by simply omitting the whiskey and using milk instead.