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1
Combine the milk, sugar, cinnamon stick, and vanilla extract in a large saucepan over medium-high heat. Bring to a boil, then reduce the heat to medium-low and simmer for 5 minutes. Remove from the heat.
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2
Dissolve the baking soda in the water and add to the milk mixture. Once the foam subsides, place the pan over medium-low heat and stir in the salt. Cook for about 45 minutes, stirring often, until the sauce thickens and turns a deep amber color. Pour the sauce into a clean bowl and let cool to room temperature.
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3
Pour the cream into a large bowl and place over an ice bath. Place a fine mesh sieve over the bowl.
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4
Whisk the egg yolks in a large bowl. Meanwhile, combine the milk, sugar, and salt in a medium saucepan over medium-high heat. Bring the mixture to a gentle simmer, then remove from the heat. Slowly add the warm milk mixture to the egg yolks, whisking until well combined. Transfer the mixture back to the saucepan and cook over medium-low heat for 5 to 10 minutes, stirring constantly, until the mixture is thick enough to coat the back of a wooden spoon. Strain the custard through the sieve and into the heavy cream. Stir for about 5 minutes, until the mixture is cool.
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5
Place the bananas in the bowl of a food processor and pulse until smooth. Stir the bananas and vanilla extract into the ice cream custard. Cover the bowl with plastic wrap and chill in the refrigerator for at least 4 hours, or overnight.
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6
Pour the custard into an ice cream maker and process according to the manufacturers' instructions. Layer the ice cream in an airtight container, alternating the ice cream with the cajeta. Freeze for about 3 hours, until firm.
