Preheat your oven to 375°F (190°C). Line your baking pan with parchment paper. Lightly grease the sides. This helps the cake release easily. Set the pan aside while you prepare the batter.

In a medium bowl, combine flour, baking powder and salt. Stir gently with a spoon. Mixing dry ingredients first helps everything spread evenly later. Set this bowl aside.

In a large bowl, beat the softened butter and sugar. Mix until light and fluffy. This step adds air into the batter. It makes the cake soft and tender.

Add the egg to the butter mixture. Beat until smooth. Then pour in the milk slowly. Mix gently. Do not overmix. The batter should look creamy and smooth.

Add the dry ingredients into the wet mixture. Mix slowly until just combined. Do not overmix. Overmixing can make the cake dense instead of soft.

Gently fold in the blueberries with a spatula. Be careful not to crush them. The berries should stay whole. They will burst beautifully while baking.

In a small bowl, mix flour, sugar and cinnamon. Add cold butter cubes. Use your fingers to rub the butter into the flour. You should get small crumbs.

Spread the batter evenly in the prepared pan. Sprinkle the crumb topping on top. Bake for 40 to 45 minutes. Insert a toothpick in the center. It should come out clean.

Remove from the oven and let it cool for 15 minutes. Then move to a wire rack. Slice into squares. Serve warm or at room temperature.