1. Make the pistachio crumb: In a medium bowl, combine the flour, brown sugar, granulated sugar, pistachios and a pinch of salt. Using a pastry cutter or your fingers, cut the butter into the mixture until the butter pieces are roughly the size of peas. Cover and refrigerate until ready to use.

2. Preheat the oven to 325° and position a rack in the center. Line 12 standard-size muffin cups with liners or grease a muffin tin with nonstick cooking spray.

3. In the bowl of a standing mixer, beat together the butter and sugar at medium speed until light and fluffy, about 2 minutes. Add the egg and beat until just combined, about 1 minute. Beat in the sour cream and orange zest until just combined.

4. In a small bowl, whisk together the flour, baking powder, baking soda and salt. Add the dry ingredients to the butter mixture in two additions, beating at medium-low speed until just combined.

5. Using a rubber spatula, gently fold the currants into the batter. Scoop the batter into the prepared muffin tins, filling each tin about three-quarters full. Top with the reserved pistachio crumb and bake until a toothpick inserted into the center of the muffins comes out clean, about 20 minutes. Remove the muffins from the oven and let them sit in the pan for 5 minutes. Remove the muffins from the pan and set on a rack to cool. Serve warm or at room temperature.