FOR THE CRUMB TOPPING:* – 3/4 cup (90g) all-purpose flour – 6 tablespoons (75g) firmly packed light brown sugar – 1/8 teaspoon salt – 3 tablespoons (42g) unsalted butter, melted FOR THE MUFFINS: – 2 cups (240g) all-purpose flour – 1 cup (200g) granulated sugar – 1 teaspoon baking powder – 1/2 teaspoon baking soda – 1/4 teaspoon salt – 1/2 cup (113g) unsalted butter, melted and slightly cooled – 1/2 cup (113g) sour cream – 2 large egg – 2-3 teaspoons vanilla bean paste or extract