Preheat Oven and Prep Pan:
Heat oven to 425°F (220°C). Line a jumbo muffin tin with paper liners or coat lightly with nonstick spray.Make Streusel Topping:
Prepare the streusel: In a small bowl, combine 2/3 cup flour (83g), 1/2 cup packed brown sugar (100g), 1 1/2 tsp cocoa powder, and 4 tbsp melted butter (56g) until crumbly. Set aside to dry while you make the batter.Whisk Dry Ingredients:
In a large bowl, whisk together 2 1/4 cups flour (281g), 3/4 cup granulated sugar (150g), 1 tbsp baking powder, 1/2 tsp salt, and 2 tbsp instant coffee.Whisk Wet Ingredients:
In a medium bowl, whisk together 2 eggs (room temp), 1 cup kefir (240ml, room temp), 1/2 cup oil (120ml), and 1 1/2 tsp vanilla.Fold Wet into Dry:
Fold the wet mixture into the dry mixture gently until no flour streaks remain. Avoid overmixing.Fill Muffin Cups and Top:
Divide batter among the muffin cups, then evenly top each with streusel.Bake with Temperature Change:
Bake at 425°F (220°C) for 5 minutes. Without opening the oven door, lower the temperature to 375°F (190°C) and bake until a toothpick inserted in the center comes out clean, about 15 more minutes (total 20–25 minutes).Cool Muffins on Rack:
Once cool enough to handle, move muffins to a wire rack to cool completely.