Preheat oven to 350°F. Line a standard 12-muffin pan with cupcake liners, or spray the cups with non-stick cooking spray. Set aside.

Mix flour, sugar, baking powder, baking soda, chai spice mix, and salt in large bowl. In a separate bowl, mix mashed bananas, egg, melted butter, brown sugar, milk, and vanilla.

Stir the banana mixture into dry ingredients just until blended (do not overmix). Gently fold in the cranberries and pistachios.

Divide batter evenly into each muffin tin, filling each about 3/4 to almost full. Bake muffins until tops are golden brown and a toothpick inserted into the center comes out clean, about 20-22 minutes. Transfer muffins to rack and cool.

Combine the butter and chocolate in a saucepan over low heat. Stir until melted.

Whisk the powdered sugar into the butter-chocolate mixture. Continue whisking and gradually add boiling water until the mixture reaches your desired consistency. Add more powdered sugar if it comes out too thin, more water if too thick. Whisk in the vanilla last and remove from heat. Allow to cool slightly.

Drizzle or pipe over the cooled muffins. Return it to the heat if it thickens.