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1
Preheat the oven to 350F.
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2
Line a baking sheet with parchment paper.
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3
Combine the flour, oats, brown sugar, and ground cinnamon in a food processor and process a few times to combine.
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4
Add the butter and pulse until combined.
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5
Pat the mixture evenly into a 4-inch square on the baking sheet.
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6
Bake until golden brown and crispy, about 15 minutes.
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7
Remove and let cool.
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8
Chop the cinnamon crunch into small pieces and set aside.
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9
Combine the milk, heavy cream, cinnamon sticks, and vanilla bean and seeds in a medium saucepan and bring to a simmer over medium heat.
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10
Remove and let steep for 30 minutes.
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11
Return to the heat and bring to a simmer.
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12
Prepare an ice bath by placing a medium bowl inside a larger bowl filled half full with ice water.
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13
Whisk together the yolks and granulated sugar until pale yellow.
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14
Slowly whisk in the warm milk mixture.
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15
Remove the cinnamon sticks and vanilla bean, return the mixture to the pan, and cook, stirring with a wooden spoon, until the mixture coats the back of the spoon, 3 to 4 minutes.
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16
Strain the custard into the bowl set in the ice bath and stir until chilled.
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17
Pour the custard into an ice cream maker and freeze according to the manufacturers directions.
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18
Fold the cinnamon crunch into the soft ice cream, cover, and freeze until hardened, at least 2 hours.
