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In the bowl of a stand mixer fitted with the flat beater, combine all of the dough ingredients. Mix on low speed until homogenous, then switch to the dough hook. Knead on medium speed for 8 to 10 minutes, until the dough starts to pull away from the side of the bowl; it will remain tacky and stick to the bowl in some places.
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Transfer the dough to a lightly greased bowl or container, like a 2-quart dough rising bucket, and let it rise at room temperature for 45 minutes to 1 hour, until puffy. Transfer the container to the refrigerator to continue rising for at least 4 hours and up to overnight. When the dough is finished rising, it will have doubled in size and be much firmer and less tacky.
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Transfer the dough to a lightly floured surface and divide it in half (290g per piece). Roll each piece into a 15" log that’s about 1" wide, then use a bench knife or kitchen scissors to cut each log into fifteen 1" pieces. (Each piece should be about 19g; the pieces don't have to be perfectly even, but the more uniform they are, the more beautiful your star will turn out.) Roll into 30 neat balls.
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Line a large cookie sheet, a rimless sheet pan, or an upside-down rimmed baking sheet with a piece of parchment; lightly grease the parchment.
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Unwrap the Brie and use a sharp serrated knife to cut the rind off the top; discard the rind. Rewrap the Brie and keep it at room temperature as the bread proofs and bakes. If your Brie comes in a sturdy outer container that’s not made of plastic, wrap the bottom portion of the container completely in aluminum foil, and then lightly grease the outside. Place the foil-wrapped container in the center of the parchment. If your Brie didn’t come in a sturdy container, see “tips,” below.
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To make a 5-point star, arrange 15 balls around the Brie container in a tight circle, leaving about 1/4" to 1/2" of space between the container and the balls (they will expand as they proof and bake). Make 5 points, using 3 balls arranged in a triangle for each point, evenly around the circle. The balls should touch but don’t need to be tightly packed. (You can also make a 6-point star by arranging 12 balls more loosely around the container, then using the remaining 18 balls to make the points.)
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Cover the baking sheet and allow the dough to rise at warm room temperature for about 1 hour, until puffy. Towards the end of the rise time, preheat the oven to 425°F with a rack in the center.
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Brush the risen dough thoroughly with the egg wash, then repeat a second time. Sprinkle generously with flaky salt.
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Bake the bread for 15 to 20 minutes, until deeply golden brown. Remove the bread from the oven and lower the temperature to 300°F. Use tongs to carefully remove and discard the foil-wrapped container. Allow the bread (and oven) to cool for at least 5 to 10 minutes. (See “tips,” for details on how to pause the recipe here and then easily finish it to serve at a gathering).
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Place the room temperature wheel of Brie in the empty space in the center, then top with the jam.
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Return the pan to the oven and bake the bread with the Brie in it for 10 to 15 minutes, until the Brie is soft and gooey. Meanwhile, melt the butter and rosemary sprig on the stove over medium-low heat until the butter begins to brown and the rosemary sizzles, about 2 minutes.
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Once the bread is baked, promptly brush it with the rosemary butter. If you’d like, tuck additional rosemary sprigs decoratively around the Brie wheel to make a crown. Devour immediately, dipping the bread into the cheese or using a spoon or knife for easier serving.
