Preheat your oven to 350 degrees Fahrenheit. Spray a 9 by 13 inch baking pan generously with nonstick cooking spray. You want to coat the corners well so the honey glaze does not cause sticking later.

In a large mixing bowl, combine the yellow cake mix, sour cream, vegetable oil, and eggs. Use a hand mixer or a sturdy whisk to blend everything until the batter is smooth and no dry pockets remain. The batter will be relatively thick. That is exactly what you want for supporting the honey swirl.

Spread the batter evenly into your prepared pan. Use a spatula to smooth the surface so the topping distributes nicely across the whole cake.

Drizzle the honey evenly over the raw batter. Do not worry about perfection here. Some areas will have more honey than others, and those spots will create lovely pockets of moisture and sweetness.

Mix the brown sugar and cinnamon together in a small bowl. Sprinkle this mixture evenly over the honey. Try to cover the entire surface without creating clumps.

Now comes the most important step. Take a butter knife and gently press the honey and cinnamon topping down into the batter. You are not stirring aggressively. Instead, use a light folding motion to push some of the topping below the surface. This creates the classic swirl effect throughout the Honey Bun Coffee Cake. If you skip this pressing step, the topping will simply sit on top and may burn during baking.

Place the pan in the preheated oven and bake for 38 to 40 minutes. Start checking at the 35-minute mark. The cake is done when it springs back lightly when pressed in the center and the edges pull away slightly from the sides of the pan.

Whisk together the powdered sugar, milk, and vanilla until smooth. Adjust the consistency if needed. If it feels too thick to pour easily, add an extra splash of milk. If it is too thin, stir in a little more powdered sugar.

As soon as the cake comes out of the oven, pour the glaze over the top. Work slowly so the glaze can melt into the warm surface and settle into every crevice. Allow the cake to cool for at least fifteen minutes before slicing. This helps the layers set up just enough to cut cleanly.