Place unopened frozen squash package on a microwave-safe plate with directions side up. Cook on high, 4 to 6 minutes. Rest in microwave, 1 to 2 minutes. Carefully remove plate from microwave. Mix squash and melted butter together in a bowl.
Stir milk, eggs, sugar, flour, and vanilla extract together in a pan until combined; add squash mixture. Sprinkle cinnamon across the top.
Bake in the preheated oven until top is golden brown, about 1 hour.