1. For the sweet potatoes - Heat oven to 400 degrees. Poke sweet potatoes and place on foil-lined baking sheet. Bake until very tender adn can be squeezed easily with tongs 1 to 1.5 hours (mine seemed to take longer, but may be less with smaller potatoes). Remove from oven and cut in 1/2 lengthwise to let steam escape. Cool at least 10 minutes.
2. For the streusel - While potatoes bake, butter 9x13 baking dish. Pulse flour, brown sugar, and salt in food processor until blended (about 4 1-second pulses). Sprinkle butter pieces over and pulse until crumbly mass forms, 6-8 one second pulses. Sprinkle nuts over and pulse until combined, 4-6 pulses.
3. Once potatoes have cooled, use spoon to scoot flesh into large bowl (should be about 8 cups). Transfer half of flesh to food processor. Using spatula, break remaining potato pieces into coarse 1 inch chunks.
4. For the filling - add melted butter, salt, nutmeg, pepper, vanilla, and lemon juice to potatoes in food processor and process until smooth, about 20 seconds. Taste for sweetness, then add up to 4 tablespoons granulated sugar if necessary; add yolks. With processor running, pour half and half through feed tube and process until blended, about 20 seconds; transfer to bowl with potato pieces and stif gently until combined.
5. To assemble and bake - Pour filling into prepared baking dish and spread into even layer with spatula. Sprinkle with streusel, breaking up any large pieces with fingers. Bake at 375 until topping is well browned and filling is slightly puffy around edges, 40 to 45 minutes. Cool at least 10 minutes before serving.
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