Preheat your oven to 350°F. Grease a 12-cup nonstick muffin pan. If your pan is not nonstick, use muffin liners.
Make the streusel by whisking together everything except for the butter until evenly distributed, and then stir in the melted butter. Set aside.
In a large bowl, whisk together the flour, salt, baking soda, baking powder, and cinnamon until everything is evenly distributed.
In a medium bowl, whisk the egg until homogenous. Add the milk, melted butter, and vanilla extract. Stir to combine and then whisk in the sugar.
Pour the wet ingredients into the dry flour mixture and whisk together. Switch to a flexible spatula to finish it off. When there are almost no dry flour bits remaining, add the rhubarb and raspberries, folding them in just until they’re evenly distributed.
Divide the muffin batter evenly between the muffin cups and sprinkle with the streusel.
Bake for 20-25 minutes, until golden. A toothpick inserted in the middle should come out clean save for a couple of crumbs.
Remove to a wire rack for 10-15 minutes, then carefully remove the muffins from the pan to cool completely on the rack.
Once cooled, these muffins can be stored in an airtight container at room temperature for 2 days or frozen in a freezer bag for longer-term storage.
