In a heavy-bottomed saucepan, whisk together egg yolks and maple syrup until well combined. Strain cream mixture through a fine-mesh strainer into pot with eggs and maple syrup; discard soaked walnuts. Whisk until fully combined, then set over medium heat and cook, whisking frequently, until a custard forms on a spoon and a finger swiped across the back leaves a clean line, or until custard temperature reaches 170°F.