In a medium bowl- mix flour, sugar and salt. Then add cold, cubed butter.

Using your fingers, work the butter into the mixture until you have large crumbs. Do not make them too small, they will melt into the cake if too fine.

Chill until you are ready to sprinkle on the cake.

Preheat oven to 350°F / 177°C / Gas mark 4.

Butter and flour or spray with non-stick baking spray - an 8x8 baking dish or 8 inch cake tin with tall sides. Make a sling out of parchment paper, if desired, by placing overlapping pieces of parchment in the pan.

In a medium bowl, whisk together the flour, baking powder, baking soda and salt. Set aside.

In a separate medium bowl, cream the butter and sugar together until light and fluffy. Add the egg, then vanilla, and beat until combined.

Add 1/2 of the flour mixture, beat until just combined. Add the buttermilk, beat until combined, then add the remaining flour mixture and beat until just combined. A few small pockets of flour are ok.

Gently fold blueberries into the batter.

Batter is thick, so being gentle is important! Spoon batter into the baking dish and use an offset spatula if necessary to level the top.

Sprinkle the crumb topping evenly over the entire cake.

Bake crumb cake for 50 - 65 minutes, or until a tester comes out clean with a few moist crumbs.

Cool on wire rack completely before slicing.