Preheat oven to 400°F and line a large baking sheet with parchment.
Stir goat cheese with 1 tbsp honey, vanilla, and a pinch of salt until smooth.
Toss plum slices with granulated sugar and lemon juice; set aside to lightly macerate.
Dust the counter with flour, then roll puff pastry into an even rectangle.
Cut pastry into 12 squares and transfer to the prepared sheet, spacing slightly.
Score a 1/4-inch border on each square and dock the centers with a fork.
Spread a small spoonful of the cheese mixture in the center of each square.
Fan a few plum slices over the cheese. Brush pastry edges with heavy cream and sprinkle with coarse sugar.
Bake 16–19 minutes until puffed and deep golden at the edges.
Cool 5 minutes. Warm jam and brush lightly over plums (optional). Drizzle remaining honey, then finish with thyme and a crack of pepper.