Sieve the spelled flour into a mixing bowl. Press a small depression in the middle with your fingers and crumble the fresh yeast into it. Add the cane sugar, lukewarm oat milk, and rapeseed oil. Knead everything (ideally in a food processor) until the dough is smooth, homogeneous, and pulls away from the edges of the bowl.
Shape the dough into a ball. Make a shallow crosswise cut across the top with scissors. Cover with a cloth and let rise in a warm place for 50 minutes until the volume has roughly doubled.
For the filling, heat a small amount of oat milk in a small saucepan over medium-high heat until it boils. Stir in the cane sugar and ground poppy seeds using a whisk. Simmer for 2 minutes. Remove from the heat and let stand for 2 minutes. Stir in lemon zest.
Knead the risen dough for 1 minute until smooth. Divide into 11 equal pieces. Shape each piece into a ball, then flatten to the size of a small saucer. Place a tablespoon of poppy seed filling in the center of each. Pull up the edges of the dough, pinch together at the top, then reshape into a ball. Place seam-side down in a greased baking dish.
Cover with a cloth and let rise for 15 minutes. Preheat the oven to 175 °C (350 °F). Brush the Buchteln generously with soy cream. Bake for about 30 minutes until golden brown. Remove from the oven and sprinkle lightly with powdered sugar. Serve warm or at room temperature.