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Combine milk, sugar, baking soda, and canela in a large pot and bring to a boil over medium-high heat.
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Decrease the heat and cook for 30 minutes at a soft boil; it will reduce to about 3 cups. In a heatproof bowl, whisk the eggs yolks lightly by hand and slowly pour in 1 cup of the hot milk mixture, whisking continuously.
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Return the mixture to the pot and cook over low heat, stirring often until the mixture has thickened slightly and resembles the consistency of half- and- half, 5 to 7 minutes.
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Remove from the heat and immediately pour into a bowl placed in an ice bath to chill.
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Discard the canela and gently whisk in the rum and vanilla.
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Chill completely before serving.