Preheat the oven to 350 degrees F (175 degrees C). Grease a 12-cup muffin tin or line cups with paper liners.

Mix together buckwheat flakes, brown sugar, butter, and vanilla extract in a bowl for the streusel topping; set aside.

Combine buckwheat, gluten-free, and rice flours in a bowl with baking powder.

Combine butter and 3/4 cup brown sugar in the bowl of a stand mixer fitted with the paddle attachment; beat until smooth and creamy. Add bananas and mix until just combined and slightly chunky. Add flour mixture, a little at a time, until fully incorporated. Fold in walnuts.

Spoon about 1/4 cup of batter into each prepared muffin cup, filling each about half-way full. Top with streusel topping.

Bake in the preheated oven until tops spring back when lightly pressed, 20 to 25 minutes. Cool in the tin for 5 minutes. Transfer to a wire rack to cool completely before serving.