Preheat oven to 350 degrees. Grease an 8 x 8-inch pan and line with parchment paper, if desired.

Prepare strawberry filling – in a small saucepan whisk together ¼ sugar and cornstarch. Stir in cold water and lemon juice, add diced strawberries. Stir over medium heat, just until it reaches a light boil. Reduce heat to love and cook 1 minute more, stirring constantly.  Remove from heat and let cool.

Prepare crumb topping. Mix flour,  sugar, and butter in a bowl and blend with pastry blender or fingers. Add brown sugar, stir with a fork and set aside.

For the cake, stir together flour, baking powder, baking soda, and salt in a bowl.

With a mixer, beat butter and sugar for several minutes until pale and fluffy.  Mix in egg and vanilla and then stir in sour cream. Add the dry ingredients, alternating with buttermilk in 2 separate batches until just combined. Be careful not to overmix!

Evenly spread slightly more than half of the cake batter into a greased 8×8 baking dish. Spoon strawberry mixture over the batter layer and gently spread evenly. Dollop remaining cake batter by the spoonful over the strawberry mixture and gently spread into an even layer.  Using your fingers seems to work best for this step!  Sprinkle the crumb topping evenly over the top of the cake.

Bake at 350 for 50-55 minutes or until a toothpick inserted into the cake comes out clean.  Allow it to cool slightly before serving.