Preheat oven to 350 degrees Fahrenheit and grease an 8x8 pan or line with parchment paper. Make sure all your ingredients are set out at least 20 minutes before you start this recipe. The eggs and butter should be at room temperature. The butter for the streusel however, can be kept in the fridge until you are ready to make the topping.
In a large mixing bowl, whisk together the softened butter and sugar until light and smooth. Add in the vanilla extract, mix, then add in the eggs one at a time, whisking until well combined. Next, add the greek yogurt and mix until smooth. Lastly, add the dry ingredients (flour, salt, baking powder, and soda) to your wet ingredients. Switch to rubber spatula and mix until half way combined. Then add in your whole milk. Continue to mix until your batter forms.
In a medium bowl, combine the brown sugar, flour, cinnamon, and a pinch of salt. Then, cube the cold butter and use your hands to combine it. You want to do this pretty quickly so the butter doesn't start to melt. You want to mix it until coarse crumbs form. Not only will this be used on top of the cake, but half will go in the middle to make a delicious cinnamon layer!
Layer half of the cake batter in your prepared pan then sprinkle half of the streusel mixture on top of the batter. Add the remaining batter then top with the remaining streusel. Bake for 50-55 minutes, depending on your stove. At the 50 minute mark, insert a toothpick in the middle of the cake and if it comes out clean, it is ready!
To make the vanilla glaze, whisk together 1 cup of powdered sugar with 2-3 tablespoons of whole milk and 1 teaspoon of vanilla. Let cool slightly before putting the glaze on top, then enjoy!