Preheat your oven to 350 degrees Fahrenheit. Grease your 9-inch square pan with butter. You can also use parchment paper for easy lifting.

Mix flour, brown sugar, cinnamon, and salt. Add the cold, cubed butter to the bowl. Use your fingers to rub the butter in. The mixture should look like coarse crumbs. Put this in the fridge until needed.

In a medium bowl, whisk the flour together. Add the baking powder and the salt. Set this bowl aside for a moment.

In a large bowl, beat the softened butter. Add the granulated sugar slowly. Beat until the mixture is light and fluffy. This usually takes about three minutes.

Add the egg to the butter mixture. Mix it well. Then, stir in the vanilla extract. Scrape down the sides of the bowl.

Add half of the dry flour mixture. Pour in the milk and mix slowly. Add the rest of the flour mixture. Do not overmix the batter.

Use a spatula to fold in the berries. Be very gentle so they do not burst. This keeps the cake pretty and white.

Spread the batter into your prepared pan. Sprinkle the cold streusel topping over the top. Bake for 40 to 45 minutes.

Check the center with a toothpick. It should come out clean. Let the cake cool on a wire rack. Serve it warm or at room temperature.