Add the butter, and mix until crumbs form and the mixture is thoroughly moistened. Set aside or refrigerate while you make the muffins.
To make the muffins:
Preheat the oven to 400°F (200°C). Line or grease 12 standard muffin cups.
Whisk together the flour, sugar, baking powder, baking soda, salt, and lemon zest in a large bowl. Make a well in the center of the mixture.
In a separate bowl, combine the cooled butter, sour cream, milk, eggs, and lemon juice. Add to the well in the flour mixture.
Stir just until the batter is combined. (A few lumps or streaks of flour are fine.)
Divide the batter among the prepared muffin cups, using about 3 tablespoons of batter to fill each cup 3/4 full. Sprinkle the topping over each muffin.
Bake 14 to 18 minutes, or until the edges of the muffins are lightly browned and a pick inserted into the center comes out clean.
Cool the muffins in the pan on a wire rack for 10 minutes. Then transfer the muffins from the pan onto the rack to continue cooling.
Video
Notes
*You'll need 1 or 2 medium lemons. If you want more lemon flavor, go with the higher end of the suggested range. You can also add a pinch or two of lemon zest to the topping if you like.
Store in an airtight container at room temperature up to 3 days.