In the bowl of a stand mixer, using the paddle attachment, combine 2 1/2 cups flour, buttermilk powder, sugar, yeast, and salt.
Add water and eggs to flour mixture. Blend at low speed until moistened. Beat 3 minutes at medium speed. Switch to the dough hook and mix in the remaining flour a little at a time, to make a soft dough, add more or less flour as needed. Knead the dough for 5 minutes until smooth and elastic. Place dough in lightly oiled bowl and turn to grease top. Cover; let rise in warm place until double about 30 minutes.
Add oil to a deep fryer or large pot until it is 2 inches deep. Heat oil to 360°F.
Punch down dough. On a floured surface, roll the dough into an 18x12-inch rectangle. Use a pizza cutter to cut the dough in six rows, each 3x12-inches. Cut each row into four 3-inch squares.
Carefully drop 3 beignets in oil (see notes below) and fry until puffed and golden brown, about 3 minutes, flipping frequently. Repeat with remaining dough.
While beignets are still hot, sprinkle heavily with powdered sugar. Serve immediately.