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To make the tangzhong: In a small saucepan, combine all of the ingredients and whisk until no lumps remain.
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Place the saucepan over low heat, and cook the mixture, whisking constantly, until thick and paste-like, and the whisk leaves lines on the bottom of the pan, about 3 to 5 minutes.
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Transfer the tangzhong to the bowl of a stand mixer fitted with the dough hook.
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To make the dough: Pour the cold milk over the tangzhong to help it cool slightly.
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Weigh your flour; or measure it by gently spooning it into a cup, then sweeping off any excess. Add the remaining dough ingredients to the bowl, then mix and knead until a sticky, elastic dough forms, about 10 minutes on medium-low speed.
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Shape the dough into a ball and place it in a lightly greased bowl or back into the mixing bowl. Cover the bowl and let it rest for 1 1/2 to 2 hours at room temperature or in the refrigerator overnight. The dough will be puffy but not necessarily doubled in bulk.
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To make the filling: In a small bowl, mix the jam and flour together until homogeneous. Set aside.
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To assemble the rolls: On a lightly floured surface, roll the dough into a 10" x 13" rectangle, about 1/4" thick. (If the dough has been refrigerated overnight, it does not need to come to room temperature.) Spread the filling into a thin, even layer on the dough, leaving a 1/2" border along one long edge; an offset spatula is a helpful tool here.
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Starting with the filling-covered long side, gently roll the dough up to form a 15" log, being careful not to squeeze out the filling. Pinch the seam closed.
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Cut the dough into ten 1 1/2" pieces with a serrated knife or unflavored dental floss.
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Place the jam buns into 2 greased 8" round pans, cut-side up.
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Preheat the oven to 350°F with a rack in the center.
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Cover the pans and let the buns rise for 25 to 35 minutes, or until noticeably puffy but not necessarily doubled in size.
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Bake the jam buns for 18 to 20 minutes, or until the tops are light brown and a digital thermometer inserted into the center reads 190°F. Allow the buns to cool for 10 to 15 minutes.
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To make the topping: While the jam buns are cooling, beat the cream cheese until smooth, then add the butter and mix until completely homogenous. On low speed, gradually add the confectioners’ sugar and freeze-dried raspberry powder and mix until fully incorporated, scraping down the sides of the bowl as needed. Add the vanilla and lemon zest and mix until smooth.
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Spread the topping onto the warm buns and serve slightly warm or room temperature.
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Storage information: Store leftover jam buns in the refrigerator, covered, for up to 2 days.
