Alternate lining a muffin pan with 6 parchment liners. If you won't be resting the batter and want to bake the muffins right away, preheat the oven to 425°F / 220°C. Otherwise, preheat the oven after letting the batter rest.
In a small bowl, melt the butter, then add in the sugar, flour, and cinnamon. Use a fork to mix the ingredients together until you have formed little clumps of dough. Set aside.
Chocolate Chip Muffins
In a small mixing bowl, melt the butter then whisk in the sugar.
Whisk in the egg, vanilla, and kefir until well combined.
Add in the flour, baking powder, and salt. Use a rubber spatula to lightly fold the mixture until only a few flour streaks remain.
Add in the chocolate chips and fold just until no flour streaks remain, and the chocolate chips have been evenly distributed.
(OPTIONAL) Cover the batter and let it rest at room temperature for at least 30 minutes and up to 1 hour before baking. This will encourage a more flavorful, higher-rising muffin!
Evenly distribute the muffin batter between the six parchment liners, filling each liner to the very top. Top each muffin with some of the streusel and extra chocolate chips, if desired.
Bake the muffins for 5 minutes. Without opening the oven door, reduce the temperature to 350°F / 175°C. Bake for an additional 10-15 minutes, or until a toothpick inserted into the center of the muffins comes out clean.