Cut beef into 1-inch cubes; place in bowl. Mix in half of the brandy; let stand for 10 minutes. Trim stems off white mushrooms; set mushrooms aside.
In spice grinder or using mortar and pestle, grind porcini mushrooms until in fine powder. In shallow dish, stir together ground porcini mushrooms, salt and pepper. Drain beef; coat, a few pieces at a time, in porcini mushroom mixture.
In large skillet over medium-high heat, melt 1 tbsp of the butter; in 2 batches, sear beef cubes on all sides, about 1 minute per batch. Transfer to plate. Wipe out pan.
In same pan over medium heat, melt remaining butter; cook stemmed mushrooms and thyme, stirring, for 3 minutes. Add soy sauce, sugar and remaining brandy; cook, stirring, until mushrooms are glazed and no liquid remains, about 30 seconds. Let cool.
For each piece, thread 1 mushroom and 1 beef cube onto toothpick. Place on rimmed baking sheet. (Make-ahead: Wrap in plastic wrap and refrigerate for up to 8 hours.)
Bake in 450°F oven until heated through, about 5 minutes. Transfer to warmed serving platter; garnish with chives.