First, preheat the oven to 350°F.

Then, grease a 9-inch cake pan and set it aside.

Next, add the eggs, oil, milk, and vanilla extract to a large bowl.

Whisk until everything is well combined.

In another bowl, mix the flour, granulated sugar, baking soda, and salt.

Then, add the dry mixture to the wet ingredients and mix just until combined.

Next, add the brown sugar and cinnamon to a separate bowl.

Mix until evenly combined.

Pour half of the cake batter into the prepared pan.

Then, spread it evenly across the bottom.

Sprinkle half of the cinnamon-brown sugar mixture over the batter. Next, gently spread it into an even layer with a fork.

Carefully spoon the remaining batter over the cinnamon layer.

Then, gently spread it without disturbing the filling underneath.

Lastly, sprinkle the remaining cinnamon-brown sugar mixture evenly over the cake.

Bake for 40-45 minutes. The cake is ready when a toothpick inserted into the center comes out clean or with a few moist crumbs.

Finally, let the coffee cake cool slightly before slicing.

Serve it warm or at room temperature with coffee or tea!