Prepare the crumble first by melting butter in a microwave-safe, medium-large bowl for 60-90 seconds on high power (cover the bowl with a damp paper towel before melting.)
Add the sugar and flour and stir with a fork until well-mixed and nice size crumbs form. Set aside.
See Café Tips above in the post for more detailed instructions and tips to ensure success.
To make these muffins gluten-free, sub an equal amount of cup to cup gluten-free flour for the all-purpose flour. I used King Arthur Measure to Measure Gluten-Free Flour with really good results. Be sure to allow the batter to rest in the pan as directed before baking. Add around 5 minutes onto the baking time or until the muffins are nicely golden and feel firm when gently touched on the top.