Ingredients
180 g all-purpose flour
100 g white sugar
5 g baking powder
3 g baking soda
3 g salt
0.5 g ground nutmeg
115 g cold unsalted butter, cut into cubes
120 ml buttermilk
1 large egg
2 tbsp lemon zest
45 ml lemon juice
5 ml vanilla extract
240 g blueberries
25 g cornstarch
15 ml water
25 g brown sugar
Directions
Step 1
In a saucepan over medium heat, combine blueberries, 25 g white sugar, and 15 ml lemon juice. Cook, stirring frequently, until blueberries release their juices. Mash the blueberries with a potato masher until broken up. Bring the mixture to a simmer, then reduce heat to low.
Step 2
In a small bowl, mix cornstarch and water until combined. Add to the blueberry mixture, stirring constantly, and cook until thickened (about 1-2 minutes). Transfer the blueberry filling to a shallow bowl and refrigerate while preparing the rest of the cake.
Step 3
For the crumb topping, combine 60 g flour, brown sugar, 0.5 g salt, and 0.5 g nutmeg in a food processor. Pulse to combine. Add 30 g cold butter and pulse until the mixture resembles coarse sand. Transfer the mixture to a small bowl and press it together with your hands to form medium to large-sized crumbs. Refrigerate the crumbs while preparing the cake.
Step 4
Preheat the oven to 175°C. Line an 20x20 cm square pan with parchment paper, leaving an overhang on all sides.
Step 5
In the food processor, combine 180 g flour, 100 g white sugar, baking powder, baking soda, 3 g salt, and 0.5 g nutmeg. Pulse to combine. Add 115 g cold butter and pulse until the mixture resembles coarse crumbs.
Step 6
In a small bowl, mix buttermilk, egg, lemon zest, lemon juice, and vanilla until combined. Add this mixture to the food processor and pulse until just combined, with no dry bits of flour remaining.
Step 7
Pour about 2/3 of the batter into the prepared pan and smooth it into an even layer. Spread the blueberry filling evenly over the batter. Place dollops of the remaining batter over the blueberry filling and spread to cover most of the filling with a thin layer of batter. (It's okay if not all the blueberry filling is covered.) Sprinkle the refrigerated crumb topping evenly over the batter and lightly press the crumbs down.
Step 8
Bake for 45-55 minutes or until the middle of the cake springs back lightly when touched. Cool the cake in the pan for about 10 minutes, then transfer it to a wire rack using the parchment overhang to cool completely (about 20 minutes). Cut into 12 pieces.