Dice half a stick of ice cold butter in to small pieces.
Combine flour, sugar, cinnamon and salt in a medium bowl.
Work butter in to flour with a fork or your hands until the mixture resembled course sand. Toss the whole bowl in to the freezer for a few minutes if the butter starts melting. When done, place bowl in freezer while you prepare the cake.
Preheat oven to 375°F.
Using an electric mixer in a medium bowl, beat sugar and lemon zest for a minute then add the softened butter. Beat until butter is well incorporated.
Add one whole egg, one egg white, and vanilla and beat until mixture gets thick and increases in size - a few minutes.
Combine the flour, baking soda and salt in a separate bowl.
Slowly add the flour mixture then milk while beating at medium low speed. The batter should be thick and heavy. Clean off any batter stuck to the beaters and add back to the bowl.
Using a silicone spatula or wooden spoon, carefully fold the blueberries in to the batter.
Transfer and even out batter to an 8x8 cake pan.
Layer the cold crumb mixture on the top of the cake pressing down gently to fill in any air gaps. Use a fork to prick the crumb and give back some texture.
Bake for 45 minutes until toothpick comes out clean.
Allow to cool completely before serving. Up to 2 hours.
Blueberry Bourbon Syrup
Add all ingredients to a medium pot and simmer over a low flame for about 15 minutes.
Stir occasionally to break up the berries.
The syrup should thicken and the heat of the bourbon should subside.
Strain syrup in to a pourable container and allow to chill before serving.
Blueberry Crumb Cake with Blueberry Bourbon Syrup by Eat Up! Kitchen at https://eatup.kitchen/blueberry-crumb-cake-blueberry-bourbon-syrup/