Preheat the oven to 425℉ or 220℃. Line a muffin tin with muffin cups before setting aside.
In a large bowl, mix together the melted butter, strawberry Kefir (or thinned Greek yogurt), sugar, eggs and vanilla extract.
Next, fold in 2 cups of the all purpose flour, baking powder and salt until just combined. Be sure not to overmix.
Slowly fold in the chopped chocolate (or chocolate chips), chopped strawberries and remaining 1 tablespoon of all purpose flour.
Scoop the batter into the prepared muffin cups, filling just shy of the top of each cup.
In a small bowl, combine the melted butter, flour and sugar for the streusel topping until it comes together in small clumps. Sprinkle the top of each muffin cup with a bit of this topping.
Bake for 5 to 7 minutes at 425℉, then, without opening the oven door, reduce the heat to 350℉ (or 180℃) and bake for another 15 minutes or until a toothpick inserted into the center of the muffin comes out clean or with only a couple of crumbs.
Remove from the oven and allow to cool for 10 minutes before transferring to a rack to fully cool or a plate to serve. Muffins can be stored at room temperature for up to 3 days (if they last that long!). Enjoy!