In a small saucepan, combine 1 cup blueberries, 1/4 cup granulated sugar, and 1 tablespoon lemon juice. Cook over medium heat, stirring occasionally, until the berries release juices and the mixture begins to simmer, about 4–6 minutes.

Mix cornstarch slurry: in a small bowl, whisk 1 teaspoon cornstarch with 1 teaspoon cold water until smooth.

Once the blueberry mixture is simmering, stir in the cornstarch slurry. Continue to cook and stir until the mixture thickens to a jam-like consistency, about 1–2 minutes. Remove from heat and stir in 1/2 tablespoon butter if using. Let cool to room temperature (about 20–30 minutes) — the filling should be thick but spoonable.

Optional Topping (streusel)

Make streusel: In a small bowl, combine 1/4 cup flour, 1/4 cup packed light brown sugar, and 1/4 teaspoon ground cinnamon. Add 2 tablespoons cold cubed butter and cut it into the dry ingredients using a fork or pastry cutter until mixture resembles coarse crumbs. Refrigerate until ready to use.

Blueberry Muffin Batter & Assembly

Preheat oven and prepare pan: Preheat oven to 400°F (200°C). Line a 12-cup muffin tin with liners or lightly grease with 1 tablespoon oil. If using liners, spray them lightly with cooking spray to prevent sticking.

Dry ingredients: In a large bowl, whisk together 2 cups all-purpose flour, 3/4 cup granulated sugar, 2 teaspoons baking powder, and 1/2 teaspoon salt until evenly combined.

Wet ingredients: In a separate medium bowl, whisk 3/4 cup milk, 1 large egg, 1/3 cup melted butter (slightly cooled), 1 teaspoon vanilla extract, and 1 teaspoon lemon zest until smooth and combined.

Combine: Make a well in the center of the dry ingredients and pour in the wet mixture. Stir gently with a spatula just until combined; batter will be slightly lumpy. Do not overmix.

Fold in blueberries: Gently fold in 1 cup fresh blueberries into the batter, taking care not to crush them. If using frozen berries, fold them in frozen to reduce color bleed.

Fill muffin cups partway: Using a 1/4-cup scoop or spoon, divide batter among 12 muffin cups, filling each about 1/3 full (this is the base layer).

Add filling: Spoon about 1 tablespoon of the cooled blueberry filling into the center of each partially filled cup. Be careful not to let filling touch the edges—keep it centered.

Top with batter: Spoon additional batter over each filled cup until cups are about 3/4 full (covering the filling). Smooth lightly.

Add streusel: If using streusel, sprinkle a generous pinch (about 1/2–1 teaspoon) on top of each muffin. Optionally sprinkle coarse sugar on top for extra crunch.

Bake: Bake in the preheated 400°F (200°C) oven for 5 minutes, then reduce oven temperature to 350°F (175°C) and continue baking for 12–15 minutes, or until the muffin tops are golden and a toothpick inserted into the edge (not the filling) comes out with a few moist crumbs. The initial high heat helps create domed tops.

Cool: Remove muffins from oven and let cool in the pan for 5–7 minutes. Then transfer muffins to a wire rack to cool for at least 15 minutes so the filling sets slightly. Serve warm or at room temperature.

Storage: Store cooled muffins in an airtight container at room temperature for up to 2 days or refrigerate for up to 5 days. For longer storage, freeze individual muffins wrapped tightly for up to 2 months; thaw at room temperature or warm briefly in the microwave.