Preheat the oven to 425°F. Line a standard 12-cup muffin pan with paper liners; this recipe makes 9 to 10 muffins, so fill any empty cups halfway with water to prevent burning.

Make the streusel topping: In a small bowl, whisk together the flour, brown sugar, and salt. Pour in the melted butter and stir with a fork until coarse crumbs form. Set aside.

Make the cream cheese filling: In another small bowl, beat the softened cream cheese, granulated sugar, cornstarch, and vanilla until smooth and just combined. Set aside.

In a large bowl, whisk together the flour, baking powder, and salt. Set aside.

In a medium bowl, whisk the egg and granulated sugar until pale and well blended. Add the Greek yogurt, vegetable oil, and vanilla extract, and whisk until the mixture is smooth and light yellow.

Pour the wet ingredients into the dry ingredients and fold gently with a rubber spatula until just combined; do not overmix.

Fold in about 1/2 cup of the blueberries and half of the raspberries, stirring gently to avoid turning the batter pink. Reserve the remaining berries for topping.

Spoon about 1 1/2 tablespoons of muffin batter into each lined cup, spreading it slightly to create a shallow well in the center. Drop a slightly heaping tablespoon of cream cheese filling into the center of each.

Top each muffin with a few reserved berries, then generously sprinkle the streusel topping over the berries, pressing gently so it sticks. Add a few more of the reserved berries on top of the streusel.

Place the pan in the oven and immediately reduce the temperature to 350°F. Bake for 30 to 35 minutes, or until the tops are golden brown and a toothpick inserted into the muffin (avoiding the filling) comes out clean.

Let the muffins cool in the pan for 5 to 10 minutes, then transfer them to a wire rack to cool completely.

While the muffins cool, make the glaze: In a small bowl, whisk together the powdered sugar and 1 1/2 teaspoons of milk or cream. Add more liquid 1/2 teaspoon at a time until the glaze is pourable but still thick. Drizzle over the cooled muffins and serve.