Preheat the oven to 350° F. Line 12 muffin tins with paper liners.
To make the streusel, mix together the butter, brown sugar, flour, and orange zest until crumbly.
To make the muffins, in a bowl, whisk together the melted butter, brown sugar, sour cream (or yogurt), eggs, and vanilla. Add the flour, baking powder, and salt. Mix until just combined.
Toss the raspberries with 1 tablespoon of flour. Fold the berries and ⅓ cup of the streusel into the batter.
Divide the batter evenly among the prepared tins. Top with the remaining streusel. Bake for 22–25 minutes, until set and lightly golden.
Remove from the oven and enjoy warm with butter and flaky sea salt.
