Bring a large pot of water to boil. Add 2 tsp. salt and cauliflower; cook until just tender, 7 minutes. Drain and rinse under cold water. Repeat with broccoli, cooking for 5 minutes and rinsing. Blot both dry with paper towels.
Grease a 2-quart microwave-safe bowl with 1 Tbsp. butter. Starting with a layer of cauliflower, fill bowl tightly, alternating veggies to create layers when bowl is inverted (as pictured).
Cover bowl with waxed paper; fit a small plate on top. Weigh down plate with a few soup cans; let stand 1 hour.
Remove cans. Microwave bowl on high for 10 minutes. Remove plate and paper, place serving dish on top and carefully invert. Remove bowl and sprinkle with pepper.