In a small saucepan, bring the cream to a simmer; remove from the heat and stir in the chocolate and butter until melted and well combined. Transfer to a small, shallow-sided dish and set in the fridge until firm.
Cut a slit into each date and carefully remove the pits. With a teaspoon, shape 1/8 of the chocolate mixture into an oblong ball. Gently squeeze a date open, and then stuff the truffle ball inside. Repeat with remaining truffle mixture and dates. Arrange the dates, truffle side up, on a plate. Sprinkle the truffle-filled portion of the dates with chopped pecans. Keep the dates refrigerated until needed.
Make the mango sauce by pureeing all ingredients in a food processor. Serve the sauce in a bowl alongside the dates for dipping.