In a small bowl, add the brown sugar, chopped almond slivers, flour, and cinnamon and whisk together until well mixed. Pour in the melted butter and stir. Make sure the streusel mixture is evenly moist and clumps up into small pieces. Use a small spoon to add the streusel topping to the muffins.
Bake muffins for 20-25 minutes, or until a tester inserted into the center comes out clean and the tops are golden brown and semi-firm to the touch. Let the muffins remain in the pan for a few minutes, and then transfer to a wire rack to let cool completely for storing.
Store at room temperature for 3 days, or freeze for up to a month.