Brown the butter: In a small saucepan over medium heat, melt the butter. Continue cooking, stirring frequently, until it foams and the milk solids turn golden brown and smell nutty. Immediately transfer to a heatproof bowl to stop cooking and let cool slightly.

Preheat oven and prepare the pistachios: Preheat to 425°F (220°C). Line a muffin tin with paper liners. As the oven heats, finely chop the pistachios -- either by hand, or ideally, by pulsing in a food processor until mealy but not pasty, with some small chunks remaining.

Make the streusel: In a small bowl, combine 1/4 cup flour, 1/4 cup sugar, and 1/4 cup of the chopped pistachios. Using a fork, stir in 2 tablespoons of the melted brown butter until crumbly. Set aside.

Mix dry ingredients: In a medium bowl, whisk together the remaining flour, baking powder, baking soda, and salt.

Mix wet ingredients: In a large bowl, whisk together the remaining brown butter, remaining sugar, eggs, sour cream, milk, and vanilla extract until smooth and combined.

Combine: Add dry ingredients to wet and fold until just combined (don’t overmix). Fold in the remaining chopped pistachios, reserving a handful or so for the top, if desired.

Fill muffin tins: Spoon batter into liners, filling each about halfway. If using pistachio cream, add one frozen portion to the center of each and cover with remaining batter to fill. For best results, fill every other muffin tin space to allow the tops to spread and dome as much as possible. Top generously with streusel and additional chopped pistachios.

Bake: Bake at 425°F for 5 minutes, then reduce temperature to 350°F (175°C) without opening the oven door and bake for another 12–15 minutes, or until a toothpick inserted into the muffin (avoiding the filling) comes out clean.

Cool: Let cool in the pan for 5 minutes before transferring to a wire rack. Enjoy warm or at room temperature.