Wash, peel, and dice the apple into small cubes. Put the diced apples into a small bowl, then toss them with the cinnamon. Set aside.
In a medium sized mixing bowl, melt the butter. Whisk in the sugar, egg, vanilla and kefir until well combined.
Add in the flour, baking powder, baking soda, and salt. Use a rubber spatula to lightly fold the mixture until just a few flour streaks remain.
Add the cinnamon apples into the batter and gently fold them in just until no streaks of flour remain.
Evenly distribute the muffin batter between the six parchment liners. Top each muffin generously with the cinnamon oat streusel, slightly pushing the streusel down to help it stick to the batter.
Bake the muffins at 425°F for 5 minutes. Without opening the oven door, reduce the temperature to 350°F / 175°C. Bake for an additional 12-15 minutes, or until a toothpick inserted into the center of the muffins comes out clean.
Serve and enjoy!