Blitz two thirds of the pistachios (½ cup) in a blender until a fine powder forms.
Mix together the pistachio powder, flour, sugars and sea salt in a bowl.
Add ¾ of the melted butter and mix with a fork or your hands. Add butter a little at a time until the crumble can be pinched together and hold its shape. You may not need all the butter.
Roughly chop the remaining pistachios and mix them through the crumble. Set aside until muffin batter is ready.
Make the Pistachio Muffins
Preheat oven to 425°F / 220°C and line a muffin pan with paper liners.
Whisk together egg, white sugar and brown sugar, just until combined.
Add sour cream, vegetable oil, pistachio paste, vanilla and almond extract and whisk together.
Add flour, baking powder and salt and whisk until lump free.
Add chopped pistachios to the muffin batter and mix through until evenly distributed.
Scoop batter into prepared muffin pans and fill almost to the top, leaving about ⅛ inch space at the top.
Generously cover the top of each muffin with the pistachio crumble.
Bake at 425°F / 220°C for 5 minutes, then turn the temperature down to 350°F / 180°C for a further 13 minutes. Do not open the oven when you lower the temperature.
Remove from the oven and let cool for 5 minutes in the pan before transferring to a wire rack to cool.
Notes
If you can't find pistachio paste you can just make your own! Add 2 cups of shelled, roasted, unsalted pistachios to a food processor and blend for 1 minute. You should see a fine powder form - keep going and eventually, it will turn to pistachio butter!