Heat oven to 350°F. Lightly butter a 9×13-inch baking pan.
Unroll phyllo. Separate sheets and loosely accordion-fold each into ruffles. Nestle the ruffles in the pan in snug rows.
Bake the dry phyllo for 15 minutes, rotating the pan once, until lightly crisped.
Drizzle melted butter evenly over the hot phyllo. Return to the oven and bake 10 minutes, until edges are golden.
Whisk eggs, 0.75 cup sugar, salt, and cinnamon until smooth. Stream in milk, cream, and vanilla; whisk until combined.
Pour the custard slowly over the hot phyllo, letting it seep into the gaps.
Bake 22–25 minutes more, until puffed and just set in the center.
Simmer 0.75 cup sugar and 0.75 cup water for 5–7 minutes. Stir in lemon juice and let the syrup cool.
Rest the cake 5 minutes. Spoon the cooled syrup evenly over the hot cake and let it absorb 15 minutes.
Dust with powdered sugar, slice, and serve warm.