In a small microwave safe bowl, melt the butter. Add in the brown sugar, flour, and coffee grounds. Mix around with a fork until you have small and large chunks of streusel. Set aside.
Mocha Muffins
Preheat the oven to 425°F / 220°C. Line a muffin pan with 6 parchment liners, alternating the liners between the 12 muffin cavities.
In a medium sized microwave safe mixing bowl, melt the butter. Add the sugar, egg, kefir, vanilla, and instant espresso into the melted butter and whisk until well combined.
Add in the flour, baking powder, and salt. Gently stir the mixture together until there are only a few flour streaks left.
Add in the chocolate chips and fold them in just until no flour streaks remain.
Evenly distribute the muffin batter between the six parchment liners. Top each muffin with coffee streusel.
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 10-12 minutes, or until a toothpick inserted into the center of the muffins comes out clean.
Espresso Glaze
In a small bowl, whisk together the powdered sugar, instant espresso, and milk. Add more powdered sugar or milk as needed to get your desired consistency. Once the muffins have cooled to room temperature, drizzle the glaze over them.