In a medium bowl, combine all-purpose flour, granulated sugar, and a pinch of salt. Drizzle in melted unsalted butter, mixing with a fork or your fingertips until the mixture resembles coarse crumbs. Set aside your streusel to allow it to firm up while you focus on the cream cheese filling and muffin batter.

In a mixing bowl, blend softened cream cheese, granulated sugar, cornstarch, and vanilla extract using an electric mixer. Beat until the mixture is smooth and creamy without any lumps, about 2-3 minutes.

In a large bowl, sift together the all-purpose flour, baking powder, and light brown sugar. In a separate bowl, whisk the egg and granulated sugar until pale and fluffy. Gently stir in Greek yogurt, vegetable oil, and vanilla extract. Combine the wet and dry ingredients, folding until just mixed.

Carefully fold in half of your fresh mixed berries, allowing them to distribute without breaking apart. Reserve the rest of the berries for topping later.

Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners. Fill each cup halfway with muffin batter, then add a generous dollop of the cream cheese filling in the center. Top with the remaining berries and sprinkle your prepared streusel mixture over each muffin.

Place the muffin tin in the preheated oven and bake for 30-35 minutes, or until the tops turn golden brown and a toothpick inserted into the center comes out clean.

Once baked, remove the muffin tin from the oven and let it cool for about 10 minutes. Gently transfer the muffins to a wire rack to cool completely.

If desired, prepare a simple glaze by mixing powdered sugar with a splash of milk or water, then drizzle it over your cooled muffins.