Add about 1/4 of the flour mixture and stir on low speed until just combined. Add about 1/3 of the buttermilk and stir.  Continue to alternating until all are added & combined.

To make streusel, whisk together flour, sugars, and salt. Drizzle in melted butter and use a fork to stir/toss together until mixture is clumpy but combined (all the butter should be absorbed).