In a medium bowl, whisk together the mustard, allulose honey, olive oil, garlic powder, salt, pepper, and cayenne.
Place the chicken thighs in a large bowl. Pour about half of the honey mustard over the chicken and toss to coat. Let marinate 1 to 2 hours in the fridge.
Preheat the grill to medium high and brush the grates lightly with oil. Grill the chicken until the center reaches 165ºF on an instant read thermometer, about 6 to 7 minutes per side.
Brush with a little reserved honey mustard sauce, and grill another minute or so. Then remove to a plate and brush with more sauce, as desired.
