Preheat the oven to 350°F (175°C). Grease and line an 8x8-inch (20x20 cm) square baking pan with parchment paper leaving an overhang for easy removal.

Dry ingredients

In a medium bowl whisk together 2 cups all-purpose flour, 2 tsp baking powder, and 1 tsp fine sea salt. Set aside.

Cream butter and sugar

In a large bowl using a hand mixer or stand mixer fitted with a paddle, beat 1 stick softened unsalted butter and 1 cup granulated sugar on medium-high speed for 2–3 minutes until light and fluffy.

Add eggs and vanilla

Add 2 large eggs, one at a time, beating briefly after each addition until combined. Scrape down the bowl and mix in 1 tsp vanilla extract.

Incorporate yogurt and milk

Reduce mixer speed to low. Add 1 cup plain Greek yogurt and 1 tbsp whole milk, mixing until smooth and uniform. The batter will be thick but moist.

Combine dry and wet

With the mixer on low, add the dry ingredient mixture in two additions, mixing just until incorporated. Do not overmix; scrape the bowl to ensure even distribution.

Blueberry Layer & Fold-ins

Macerate blueberries

Place 1.5 cups fresh blueberries in a small bowl. Add 2 tbsp granulated sugar, 1 tsp lemon zest, and 1 tbsp lemon juice. Toss gently and let sit for 10–15 minutes to draw out juices.

Make quick blueberry compote

Transfer the macerated blueberries and their juices to a small saucepan and warm over medium heat until simmering. In a small cup stir 1 tsp cornstarch with 1 tsp cold water until smooth, then whisk into the simmering blueberries. Cook 1–2 minutes until slightly thickened. Remove from heat and cool for 5 minutes.

Fold blueberries into batter

Reserve about 1/4 cup of the compote or whole berries for topping. Gently fold the remaining blueberries (most will be intact) into the prepared cake batter using a spatula — distribute evenly but avoid smashing the berries. This will create bits/pieces of blueberry throughout the cake.

Assemble & Bake

Spread half of the batter into the prepared 8x8 pan, smoothing to the corners with an offset spatula. Spoon the reserved blueberry compote/berries in an even layer over the batter (this creates a visible blueberry piece layer). Dollop the remaining batter over the compote and gently spread to cover as evenly as possible. If desired, press 1/4 cup extra fresh blueberries on top for decoration.

Bake in the preheated 350°F (175°C) oven for 35–45 minutes, or until the top is golden and a toothpick inserted into the center (avoiding large berry pockets) comes out clean or with a few moist crumbs. If the top browns too quickly, tent loosely with foil for the last 10–15 minutes.

Remove from oven and place pan on a wire rack. Let cake cool in the pan for 20 minutes, then use the parchment overhang to lift it out and transfer to the rack to cool completely (another 30–45 minutes) before glazing or slicing.

Glaze & Serve (optional)

Make glaze

Whisk together 1/2 cup powdered sugar, 1 tsp fresh lemon juice and 1 tsp milk or water until smooth and pourable. Add a few drops more liquid if needed to reach drizzling consistency.

Finish

Drizzle glaze over the cooled cake and scatter the reserved 1/4 cup extra blueberries on top. Let glaze set for about 10 minutes. Slice into 9 pieces (3x3) or 12 smaller pieces and serve.