Preheat oven to 425°. Heat a small frying pan over medium heat and add butter and 1 tbsp. tea. When butter foams, remove from heat, cover, and let infuse 10 minutes. Strain butter through a fine sieve; discard tea.
Meanwhile, using a large, sturdy, sharp knife, cut off stems and seedless "necks" of squashes (save seeded parts for another use). Stand each neck on a flat side and slice peel off with 7 or 8 cuts, leaving a kind of octagonal shape. Cut necks into 1/2-in. slices. Lightly oil 2 baking sheets and place squash, slightly separated, on sheets.
Pulverize remaining 1 1/2 tsp. tea leaves (if already fine, skip this step). Mix with salt.
Brush infused butter onto tops of squash slices, then season with pepper and rooibos salt. Bake until very soft, about 25 minutes. Transfer to a platter and sprinkle with chives.