Preheat oven to 350 degrees F. Line a 9x13 baking pan with parchment paper and spray with cooking spray.
Make the espresso hazelnut streusel. Combine all of the ingredients including flour, brown sugar, melted butter, hazelnuts, instant espresso, cinnamon, vanilla, and salt, until well combined. Set aside.
Prepare the chocolate banana kefir cake. Cream together the softened butter with sugar until light and fluffy, about 3-4 minutes. Add the eggs, mashed bananas, and vanilla, mixing until well combined.
Combine the dry ingredients together including the flour, baking powder, baking soda, and salt. Add the dry ingredients to the batter alternatively with the kefir, starting and ending with the flour mixture.
Toss the chocolate chips with 1 tablespoon flour. Add the chocolate chips to the cake batter, gently folding them in. Spread the cake batter in the prepared baking dish in an even layer. Top the cake with the streusel.
Bake the chocolate banana kefir cake for 45 minutes or until a toothpick inserted in the center comes out clean. Let cool completely.