Preparation Preheat oven to 325°F. Spread out pecans on a large rimmed baking sheet and toast until slightly darkened in color and fragrant, 13–15 minutes. Let cool. Bring milk and poppy seeds to a boil in a medium saucepan; reduce heat to medium-low and simmer, stirring occasionally, until milk is almost completely evaporated, 13–15 minutes. Let mixture cool. Scrape into a food processor. Cut 6 Tbsp. butter into pieces and add to food processor along with pecans, brown sugar, salt, and vanilla. Process until a paste forms. Melt remaining 4 Tbsp. butter in a small saucepan over medium heat (or in a microwave). Line a rimmed baking sheet with parchment, then brush with some of the melted butter. Place a stack of 6 phyllo sheets on a work surface. Cover with plastic wrap, then a damp kitchen towel. Arrange prepared baking sheet so a long side is facing you and stack 2 phyllo sheets on top; brush lightly with butter. Top with another sheet of phyllo and brush lightly with butter. Repeat with 2 more sheets, then top with final sheet; do not brush with butter. Using an offset or rubber spatula, spread poppy seed mixture over pastry in an even layer, leaving a 1" border on all sides. Starting with the long side closest to you, gently roll up pastry and turn seam side down; tuck exposed ends underneath. Using a sharp knife, cut horizontal slits about halfway through roll, spacing every 1" along length. Brush with egg. Bake strudel until golden and crisp, 40–45 minutes. Transfer baking sheet to a wire rack and let strudel cool. Just before serving, dust with powdered sugar and slice. Do Ahead: Strudel can be baked 1 day ahead. Let cool completely, then cover with plastic wrap and store at room temperature.
Preparation Preheat oven to 325°F. Spread out pecans on a large rimmed baking sheet and toast until slightly darkened in color and fragrant, 13–15 minutes. Let cool. Bring milk and poppy seeds to a boil in a medium saucepan; reduce heat to medium-low and simmer, stirring occasionally, until milk is almost completely evaporated, 13–15 minutes. Let mixture cool. Scrape into a food processor. Cut 6 Tbsp. butter into pieces and add to food processor along with pecans, brown sugar, salt, and vanilla. Process until a paste forms. Melt remaining 4 Tbsp. butter in a small saucepan over medium heat (or in a microwave). Line a rimmed baking sheet with parchment, then brush with some of the melted butter. Place a stack of 6 phyllo sheets on a work surface. Cover with plastic wrap, then a damp kitchen towel. Arrange prepared baking sheet so a long side is facing you and stack 2 phyllo sheets on top; brush lightly with butter. Top with another sheet of phyllo and brush lightly with butter. Repeat with 2 more sheets, then top with final sheet; do not brush with butter. Using an offset or rubber spatula, spread poppy seed mixture over pastry in an even layer, leaving a 1" border on all sides. Starting with the long side closest to you, gently roll up pastry and turn seam side down; tuck exposed ends underneath. Using a sharp knife, cut horizontal slits about halfway through roll, spacing every 1" along length. Brush with egg. Bake strudel until golden and crisp, 40–45 minutes. Transfer baking sheet to a wire rack and let strudel cool. Just before serving, dust with powdered sugar and slice. Do Ahead: Strudel can be baked 1 day ahead. Let cool completely, then cover with plastic wrap and store at room temperature.