Preheat the oven to 350°F.

Spray a 9-inch square pan liberally with nonstick cooking spray. Set aside.

Place the butter, sweetener, and vanilla into a large mixing bowl. Use a mixer to cream ingredients together on medium-high speed until smooth, 1-2 minutes. Add the eggs and sour cream and continue beating until mixture is light and fluffy, about 1 minute.

Sift the flour, baking powder, baking soda, and salt into the bowl. Mix on low speed, scraping down sides of bowl with rubber spatula as needed, until a smooth, thick batter forms.

Pour the batter into the prepared pan and smooth it into an even layer.

Place the flour, sweetener, cinnamon, and salt in large mixing bowl. Stir with a fork to combine. Drizzle the melted butter into the bowl and mix with fork until mixture forms large crumbs. Sprinkle the crumble evenly over the cake batter.

Bake the coffee cake on middle rack of oven until topping is golden and a toothpick inserted in the center comes out clean, 35-45 minutes.

Check to see that coffee cake is done. Remove from oven or add time as needed.

Allow cake to cool in pan for 15-20 minutes before serving. To store, cover tightly with aluminum foil or store in airtight container and keep at room temperature for 1-2 days, refrigerate for up to week, or freeze for up to 2-3 months.