Melt 1 tablespoon of butter in a medium non-stick skillet over medium-high heat. Add the squash and sauté until lighter in color and soft, but not turning to much, about 5 minutes. Remove from the heat and cool.
In a medium mixing bowl, stir together the squash and egg. Add the remaining ingredients and stir to combine.
Coat a large cast iron skillet with a generous layer of olive or canola oil, and set it over high heat. Test the heat with the first latke: add a tablespoon full of batter to the pan. The latke should immediately sizzle and firm. Cook for one minute on each side. Remove to a plate with paper towels. Repeat with the remaining batter, making sure not to crowd the pan.
In the meantime, combine the yogurt and the cumin. Salt to taste.
To serve, dollop each latke with a little yogurt, and garnish with a cilantro leaf.
Serve immediately, or cover and refrigerate. Simply reheat the cakes in a 400 degree oven for 5-10 minutes until crisp.