Preheat your oven to 400°F (200°C). Line a muffin tin with 6 paper liners.

In a small bowl, mix melted butter, flour, brown sugar, salt, and spices until crumbly. Set aside in the refrigerator.

In a medium bowl, whisk together flour, baking powder, salt, cinnamon, and nutmeg.

In a large bowl, whisk together the egg and granulated sugar. Stir in melted butter, sour cream, milk, and vanilla extract.

Fold dry ingredients into wet ingredients gently. Let rest for 10 minutes.

Prepare rhubarb by cutting into small pieces and toss in a bit of flour.

Fold floured rhubarb into muffin batter gently.

Divide batter among the muffin cups about two-thirds full. Top with streusel.

Bake for 5 minutes at 400°F, then reduce to 350°F and bake for an additional 12-15 minutes.

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