Preheat your oven to 350 degrees F and line two baking sheets with parchment paper or silicone baking mats.
Make the cookie dough by combining the vanilla cake mix, flour, cornstarch and cinnamon in a medium bowl and whisk until everything is evenly combined.
Add the eggs, melted butter and sour cream to the dry ingredients and mix until a thick dough forms.
Use a 2 tbsp cookie scoop to scoop the cookie dough onto the prepared baking sheets, then use your thumb to make an indentation in the middle of each cookie dough ball.
For the streusel topping, combine flour, brown sugar and cinnamon. Pour in the melted butter and mix until you’re left with a slightly wet, crumbly streusel mixture.
Add 1 tbsp of the streusel topping to the middle of each cookie then bake for 9-11 minutes or until the cookies are set and the bottoms are a light golden brown.
Once the cookies are cooled, combine the icing ingredients and whisk until smooth. You may need to add more heavy cream to get a thinner consistency for drizzling.
Drizzle the icing on top of each cooled cookie, then let the icing set up for about 30 minutes before enjoying!