Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking dish.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt until well combined.
Add the softened butter, egg, vanilla extract, and sour cream to the dry ingredients. Mix with an electric mixer on medium speed until the batter is smooth and well combined, about 2-3 minutes.
In a separate bowl, combine the ground cinnamon, brown sugar, and chopped pecans (if using). Mix until evenly distributed.
Pour half of the batter into the prepared baking dish and spread evenly. Sprinkle half of the cinnamon sugar mixture over the batter. Pour the remaining batter on top and spread it out gently. Then add the rest of the cinnamon sugar mixture on top.
Using a butter knife or a skewer, gently swirl through the batter, creating a marble effect without fully mixing it in.
Place in the preheated oven and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
Let the cake cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.
Slice into squares and serve warm or at room temperature, enjoying with a fresh cup of coffee!