Cream together the butter, cream cheese, and sweetener with an electric mixer. Add in the eggs and vanilla and mix. Add in the rest of the dough ingredients and mix until incorporated.
Spread in the prepared baking dish. Top with the berries. Drizzle the melted butter over the cake and sprinkle with the powdered sweetener.
Bake for 35-40 minutes until the cake is barely golden and no longer looks wet on top. Do not over bake, or you will lose the gooey butter cake texture.
Store at room temperature for 2-3 days. For longer storage, the cake can go in the refrigerator, but pieces need to be warmed before serving for the right texture.
Notes
Nutrition: this recipe has 15 servings. There are 4.3 NET carbs per serving.Substitutions: swap the blackberries for raspberries, blueberries, or strawberries. It's also fine to use a combination of berries. To Reheat: warm on a microwave safe plate for 10-15 seconds. Don't overheat, or it will become hard. To Freeze: store in a freezer safe container for up to 2 months. Defrost in the fridge and reheat in the microwave before serving.