Preheat your oven to 400°F (200°C) and line a six-cup muffin tin with paper liners.

In a small bowl, combine flour, dark brown sugar, and softened butter for the streusel topping, mixing until crumbly.

In a medium bowl, whisk together all-purpose flour, salt, baking powder, baking soda, cinnamon, and ginger. Reserve two tablespoons of this mixture for coating the fruit.

In a large bowl, whisk together granulated sugar, buttermilk, egg, and melted butter until smooth.

Fold the dry ingredients into the wet mixture gently until just combined.

Coat the diced rhubarb and raspberries with the reserved flour mixture before folding them into the batter.

Fill the muffin cups about two-thirds full with batter and sprinkle streusel topping over each.

Bake for 18-20 minutes or until golden brown and a toothpick comes out clean.

Cool in the tin for a few minutes, then transfer to a wire rack to cool completely before serving.