Make a lengthwise slit in each date. Squeeze date open and remove pit. Knead marzipan to soften. Shape level teaspoon marzipan into an oblong ball. Stuff into centre of date. Gently squeeze date shut, leaving some marzipan still exposed. Repeat with remaining dates.
Melt chocolate in a small bowl over simmering water. Dip each stuffed date half-way into the melted chocolate. Set on parchment paper-lined baking sheet, stuffed-side up. While chocolate-dipped side is still wet, sprinkle with pinch of chopped pistachios. Repeat with remaining dates.
Refrigerate dates to set chocolate. Keep chilled until ready to serve.