Preheat your oven to 325 degrees F and grease a 9x13 cake pan with baking spray. Set the pan aside for now.
Add the vanilla cake mix, eggs, oil, and Greek yogurt to a large bowl and whisk everything together well, making a smooth, thick batter.
1 (15 ounce) box of yellow cake mix, 4 eggs, ¾ cup vegetable oil, 1 cup plain Greek yogurt
Pour half of the cake batter into the greased cake pan and spread it across the bottom of the pan.
In a separate small bowl, stir together the brown sugar and cinnamon. Sprinkle this mix over the cake batter in the pan, covering the cake batter evenly.
1 cup dark brown sugar, 1 Tablespoon ground cinnamon
Gently spread the remaining cake batter in the pan, layering it on top of the cinnamon sugar sprinkle.
Use a butter knife to gently swirl the cinnamon sugar crumble into the cake batter. You want to just run the knife through the batter once or twice, making a nice, simple swirl. You do not want to stir it too much.
Bake the cake in the preheated oven for 30 minutes. The center of the cake should spring back to the touch and the edges of the cake will be pulled away from the sides of the pan.
Remove the cake from the oven and keep it in the pan while you make the frosting.
In a small bowl, whisk together the powdered sugar, whole milk and vanilla extract. Pour the frosting over the hot cake and spread it across the whole surface of the cake.
Sprinkle the top of the cake with chopped pecans, if using, then slice and serve while the cake is nice and warm.
½ cup chopped pecans (optional)
Notes
You can also bake this honey bun cake in a bundt pan. Grease and flour a bundt pan well before baking. Once the cake has baked and cooled for about 30 minutes, flip the cake out of the pan and drizzle the glaze over the top of the cake.