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Combine the raisins and rum in a small bowl and set aside to soak for 30 minutes.
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In a large bowl (preferably metal) or the bowl of a stand mixer, whisk the egg whites — by hand or mixer — until they form stiff peaks. Set aside.
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In a separate large bowl, whisk together the egg yolks, pancake mix, and milk until mostly smooth; a few small lumps are OK.
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Add the egg whites to the batter, folding gently just until incorporated.
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Place a 10” to 11” sauté pan (preferably non-stick with sloped sides and a lid) over medium heat. Add 2 tablespoons (28g) of the butter, letting it melt then sizzle and brown slightly.
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Pour the batter into the hot pan. Drain the raisins or cranberries and sprinkle them onto the pancake (they’ll sink).
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Cover the pan with its lid or a piece of aluminum foil and cook the pancake until the bottom is golden brown, 4 to 7 minutes; use a spatula to lift the edge of the pancake to check its color.
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Use the spatula to loosen and then slide the whole pancake onto a large plate. The top of the pancake won’t be set so the process may be a bit messy; that’s OK.
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Add the remaining tablespoon (14g) of butter to the pan and let it melt, then sizzle.
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Flip the pancake from the plate onto its uncooked side in the pan. It’s OK (really!) if the pancake tears or breaks apart during this process.
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Cook the second side of the pancake for another minute or two, until it’s just starting to color.
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Remove the pan from the heat and use two forks to tear the pancake into 1” to 3” pieces, working right in the pan.
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Generously dust the pancake with confectioners’ sugar before serving it immediately, either straight from the pan or on a serving plate. Offer sauce or compote on the side.
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Store any leftover pancake, securely covered, for a day or so in the refrigerator. Reheat briefly in the microwave or over low heat with a bit of butter in a sauté pan.
