Mix together the crumble topping in a bowl using a pastry cutter or a fork. You can also use a food processor. Mix together until the mixture resembles corse crumbs, and then place the mixture in the fridge until needed.
For The Cake
Preheat the oven to 375° and line an 8x8 pan with parchment paper.
In a large bowl, whisk the melted butter and sugar together, and then add in the milk, egg and vanilla. Mix until well combined.
Add the flour, baking powder and salt to the bowl and mix until just combined.
Fold in the fresh blueberries and pour the batter into the baking dish. Spread the batter out evenly and then top with the cold crumble. Spread the crumble out all over the cake as evenly as possible.
Bake the cake for 25-30 minutes or until a toothpick comes out clean.
When the cake is done, let it cool for 10-15 minutes and then transfer it out of the pan onto a cooling rack.
Notes
This gluten free blueberry crumble coffee cake can be made with regular flour if you don't need it gluten free.
If you don't love cinnamon, use less of it.
The cake can be made in a round or square pan.
The cold crumble is important so that is crisps up nicely. If the butter is warm, it will spread out more.