Question on the mixer used in the Biscoff Recipe

It really depends on how heavy duty your hand mixer is. If you can make other types of cooke dough with it, then it should hold up just fine for this one. If it gets too stiff for your particular model, there's nothing to say you can't use a wooden spoon or even your own two clean hands to work in any remaining flour. Sometimes using your hands is the best method of deciding when a dough is wet or dry enough to suit. In this case, the dough should peel away from your hands about as well as it would a stand mixer paddle to form the ball.