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In a bowl, whisk together sugar and eggs.
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In a medium saucepan, over medium low heat, add raspberries, lemon juice and lemon zest.
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Heat until raspberries start to break down (around 5 minutes), carefully stirring watching to ensure it doesn't burn.
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Add in the sugar mixture and constantly whisk for 2 minutes.
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Stir often until it begins to thicken (about 5 minute).
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Add butter and continue stirring often until all butter is melted.
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Over a bowl, use a strainer to press the solids out of the mixture.
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Refrigerate until cold (at least 4 hours).
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Keep refrigerated in air tight container for up to 10 days.