Special equipment: a 24-cup mini muffin tin, 24 mini muffin paper liners
In a double-boiler or a heatproof bowl set over a saucepan of simmering water, whisk together egg yolks, sugar, vanilla, half the lemon zest, and the lemon juice until well combined. Continue to whisk constantly over very low heat, until the yolk mixture has thickened and becomes pale yellow, about 3 to 4 minutes. Remove from heat. Add the sour cream, mix until just combined, and set aside.
In a large bowl, whip the heavy cream with a whisk until soft peaks form. Gently fold whipped cream into egg mixture. Gently stir in poppy seeds. Divide mixture evenly between lined mini muffin cups. If you don't have mini muffin cups, you can use regular sized to get half as many. Freeze for at least 2 hours or until firm.
To serve, peel away the paper liners and garnish with remaining lemon zest.