1. Preheat oven to 200 C and line a 12 hole muffin tin with papers.  Set aside.

To make the streusel:

1. Place the butter, brown sugar, flour and salt into a small bowl and using your fingertips mix the butter into the flour/sugar until the mixture resembles wet sand.  Set aside.

To make the cheesecake filling:

1. Place the ricotta, egg yolk, sugar, lemon zest and vanilla into a small bowl and mix to combine.  Set aside.

To make the muffins:

1. Place the butter in a small saucepan over medium heat and cook until the butter is melted, foaming and turned brown and smells nutty.  Set aside to cool slightly.
2. Place the flour, baking powder, bicarb soda and salt into a bowl and whisk to combine creating a well in the centre.
3. Into a separate bowl, place the eggs, sour cream, brown sugar, vanilla and slightly cooled brown butter and whisk to combine.
4. Pour the wet ingredients into the well of the dry and gently mix using a wooden spoon.
5. Gently fold through the mixed berries.  The batter is quite thick (especially when you add the frozen berries), if required add a tablespoon or two of milk to loosen.
6. Place a heaped tablespoon of muffin batter into each muffin paper.
7. Top with a tablespoon of cheesecake filling.
8. Cover the cheesecake layer with remaining muffin batter.
9. Sprinkle each muffin with a generous layer of streusel.
10. Bake for 15 minutes or until golden and muffins bounce back to the touch.
11. Allow to cool for 5 minutes before placing on a wire rack.