Preheat your oven to 375°F (190°C) and grease a 9x9-inch baking dish or line it with parchment paper.

In a large bowl, combine 2 cups of all-purpose flour, 1 teaspoon of baking powder, ½ teaspoon of baking soda, and ½ teaspoon of salt. Whisk to mix thoroughly.

In another bowl, mix 3 tablespoons of softened butter, 1 egg, and 1½ cups of buttermilk. Beat until smooth and well combined.

Pour the wet mixture into the bowl of dry ingredients and stir gently until just combined, being careful not to over-mix.

Pour half of the batter into the prepared baking dish, spreading it evenly across the bottom.

In a small bowl, combine 3 tablespoons of softened butter, 1 tablespoon of cinnamon, ½ cup of flour, and both types of sugar. Mix until a streusel-like texture forms.

Sprinkle the streusel filling over the first layer of cake batter, then add the remaining batter on top.

In another bowl, mix ½ cup of flour, ½ tablespoon of cinnamon, ¼ cup of cubed butter, and ⅛ teaspoon of salt until a crumbly mixture forms. Sprinkle this over the top of the batter.

Place the baking dish in the oven and bake for 25-35 minutes. Check for doneness with a toothpick.

While the cake is baking, mix ½ cup of powdered sugar with 2½ teaspoons of milk until you achieve a smooth consistency.

Let the cake cool slightly before drizzling with icing. Cut into squares and serve.