Make the sourdough starter: Warm the milk in a small saucepan over low heat to about 110 degrees F. Transfer to a medium bowl, sprinkle with the yeast and let stand until foamy, about 5 minutes. Whisk in the cider, brown sugar, butter and 1 cup flour until smooth. Cover with plastic wrap and let stand at room temperature overnight.
The next morning, make the pancakes: Whisk 1/2 cup flour, the egg, baking soda and apple into the sourdough starter. Melt 1 tablespoon butter in a cast-iron griddle or nonstick skillet over medium heat and swirl to coat. Pour about 1/4 cup batter onto the griddle for each pancake. Cook until the tops are bubbly and the edges set, about 3 minutes, then flip and cook until golden and cooked through, 1 to 2 more minutes. Repeat with the remaining batter, adding more butter as needed. Drizzle with syrup and garnish with pecans, if desired.