Adjust your oven rack to the middle position.

Preheat the oven to 350°F (175°C).

Grease a 9×13-inch baking dish or a round 9-inch springform pan. You can use butter or a non-stick spray. This prevents the cake from sticking later.

Place the softened butter (½ cup) in a large mixing bowl.

Add 1 cup of granulated sugar.

Beat with a hand mixer (or stand mixer) until the mixture is fluffy. This usually takes 2 to 3 minutes.

Scrape down the sides of the bowl so everything is well combined.

Crack in the eggs, one at a time.

Blend well after each addition.

Stir in the vanilla extract (1 teaspoon).

Scrape the bowl again to ensure all ingredients mix thoroughly.

Add the sourdough starter (1 cup) to the butter-egg mixture.

Mix on low speed until just combined.

Stop mixing when the starter is dispersed.

In a medium bowl, whisk together flour (3 cups), baking powder (2 teaspoons), baking soda (½ teaspoon), and salt (½ teaspoon).

If you plan to add 1 teaspoon of cinnamon to the batter, you can whisk it in now.

Set this bowl aside until you are ready to mix everything together.

With the mixer on low, add about one-third of the flour mixture.

Pour in about half the milk (½ cup).

Add another one-third of the flour mixture.

Pour in the remaining half of the milk (another ½ cup).

Finally, add the last portion of flour.

Mix just until you see no more streaks of flour. Over-mixing can lead to a dense cake, so stop when everything looks combined.

In a small bowl, combine ½ cup of granulated sugar, ¼ cup of brown sugar, 2 teaspoons of cinnamon, and 4 tablespoons of melted butter.

Stir well with a fork until you have a crumbly texture.

If you like nuts, fold in ½ cup of chopped walnuts or pecans.

Pour half of the batter into your greased pan.

Sprinkle half of the cinnamon-sugar topping over the batter.

Carefully pour the remaining batter over that layer.

Top with the rest of the cinnamon-sugar mix, spreading it evenly.

Use a butter knife to gently swirl the topping into the batter. This creates a marbled look in your coffee cake.

Place the pan in the preheated oven on the middle rack.

Bake for 35 to 40 minutes, or until a toothpick inserted in the center comes out clean.

Avoid opening the oven too often, as this can affect the rise.

Remove the cake from the oven.

Let it rest in the pan for about 10 minutes. This helps the structure firm up.

Transfer it to a cooling rack if you used a springform pan. If you used a 9×13-inch dish, you can serve the cake straight from the pan.

Slice and enjoy a warm piece, or let it cool completely.