Combine first four ingredients in a 1-cup measuring cup. Let stand until mixture reaches 2/3 mark. Heavily spray food processor bowl, knife and under lid with a non-stick product. Add yeast mixture, 1 cup warm water, honey, and all-purpose flour. Process in 8-10 short burst until blended. Mix buckwheat flour with salt and add with margarine. Process 10 seconds. Scrape down bowl.
Process 10 seconds 3 times, allowing a few seconds rest between spins. Spread our batter in bowl. Cover. Let rise until double in bulk. (30-35 minutes). Process 5 seconds to beat down. Transfer batter to an oval 1 and 1/2-quart oven proof casserole heavily sprayed with a non-stick product. Bang on counter 2-3 times to dislodge air bubbles. Cover with paper towel. Let rise until double in bulk. (20-25 minutes).
Bake 35-40 minutes until puffed high and a rich brown. Cool in casserole on a rack 5 minutes.