• Preheat oven to 350 degrees. Prepare an 8x8 baking dish, then line it with parchment paper for easier removal. Spray with cooking spray or oil and set aside. 
• Make buttermilk by combining the 1/2 cup milk with vinegar and set aside for at least 10 minutes.
• Make the cinnamon crumble by combining all ingredients in a food processor. Place in refrigerator until read to use. If you don't have a food processor and want to make this by hand, then grate the butter first before mixing together. 

Make the batter

• Cream butter with sugar in an electric mixer or stand mixer until pale yellow. Add yogurt or sour cream and vanilla. Add the dry ingredients (flour, baking powder and soda and salt) along with the buttermilk. Fold batter by hand until combined. Batter will look airy and shaggy.
• Spread half of the batter in the prepared pan. Sprinkle with half of the cinnamon crumble. Dollop the remaining batter on top, and using an offset spatula, spread the layer of batter over the crumble gently. Top with remaining cinnamon crumble.
• Bake for 40-45 minutes or until center springs back to touch. Allow to cool completely before adding the glaze or cutting. 
• For the glaze, combine all of the ingredients and drizzle onto the cooled cake.