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1
Place the blackberries in a medium pot, and place the blueberries in a second medium pot. Squeeze half of the lemon into each pot, place half of the vanilla bean into each pot, and sprinkle 1/4 cup sugar into each.
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2
Bring the berries to a simmer over medium heat, stirring occasionally, until the berries have completely broken down and are very soft, 15-18 minutes.
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3
Strain each mixture into separate bowls. You'll need 1/2 cup of each fruit puree. If you have more than 1/2 cup, you can return the puree to the pot and bring to a simmer. Simmer for 10-15 minutes to reduce it slightly to concentrate the flavors. The purees don't need to cool completely before you use them, but letting it cool for at least 15 minutes will make the process easier (and allow the trifles to freeze faster later)!
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4
In the bowl of an electric mixer, beat the cream and powdered sugar to medium peaks, 3-4 minutes.
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5
For each semifreddo recipe, you'll use 1/2 cup fruit puree, 1/2 cup sour cream, and half of the whipped cream mixture. In a large bowl, whisk the blackberry puree and 1/2 cup sour cream together to combine, then gently fold in half of the whipped cream. Repeat with the blueberry puree.
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6
Building the trifles is super free form; you can do it in any order you like, alternating the two flavors of semifreddo with the meringues and angel food cake. I like to start with one of the semifreddos, then alternate between the meringues and angel food cake. Spoon the first flavor of semifreddo into a medium glass, then sprinkle a layer of meringues and/or angel food cake (press them into the semifreddo a little bit). Then spoon the other flavor of semifreddo into the glass, continue this until the glass is completely full, and repeat until you've used up all your components and filled all your glasses.
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7
Freeze the trifles until the top layer is firm; this can be as little as 30 minutes and as much as 1 1/2 hours, depending on how hot it was when you built it and how cold your freezer runs. Let the trifles sit at room temperature for up to 5 minutes before serving.
