Preheat your oven to 350F. In the bowl of a stand mixer with the paddle attachment (or using a hand mixer) cream together the butter and sugar.
Add the egg and the vanilla extract. Mix until they're incorporated.
Now grab a medium size bowl and add your flour. Then add in your baking powder, salt, and cinnamon. Stir the flour mixture with a fork to blend the ingredients.
Next you're going to start adding in your flour and your milk. Add in the milk 1/4 cup at a time, alternating with the flour mixture until you've used 3/4 cup milk and all the flour mixture. You'll have a very, very thick batter.
Now here's my favorite little trick for making sure the blueberries don't all sink to the bottom when you bake the cake. Put your 2 cups of blueberries into a bowl, and sprinkle a tablespoon of flour on top. Use your hands and mix until the blueberries are coated in flour. This will help suspend them throughout the cake.
Now let's make the best part, the crumb topping! In a medium sized bowl, put your flour, sugar, cinnamon, and salt. Next add in your cold butter.
If using the fork or two knives method, I recommend cutting the butter into small pieces first. If you have a pastry cutter like above, then you can add the butter in whole. Work the butter into the flour mixture until it's all crumbly. This takes a while, but be patient and you'll get there. It's work the extra work! When it's all crumbly, sprinkle it all over your cake batter.
To make the topping for this cake I highly recommend a pastry cutter. If you don't have one, a fork or two knives will work as well, but a pastry cutter makes very quick work of making any kind of crumb topping.
This is one of my family's very favorite desserts so it rarely lasts more than two days, but it can keep 3-4 days in the refrigerator. I always recommend refrigerating baked goods that contain berries during the summer. I hope you enjoy making and eating this blueberry coffee cake as much as I do!