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.
253.54 calories; 5.55 g protein; 27.01 g carbohydrates; 0.83 g dietary-fiber; 14.84 g sugars; 14.39 g fat; 8.47 g saturated-fat; 103.91 mg cholesterol; 2568.44 IU vitamin-a-iu; 1.83 mg niacin-equivalents; 0.1 mg vitamin-b6; 2.73 mg vitamin-c; 35.86 mcg folate; 81.77 mg calcium; 1.11 mg iron; 15.34 mg magnesium; 196.8 mg potassium; 127.77 mg sodium; 0.13 mg thiamin; 129.54 calories-from-fat; 41 percent-of-calories-from-carbs; 49 percent-of-calories-from-fat; 8 percent-of-calories-from-protein; 29 percent-of-calories-from-sat-fat