In a medium bowl, whisk together the flour, baking powder, and salt.
In a large bowl, whisk together the Greek yogurt, egg, melted butter, and milk. Stir the dry mix into the wet mix until combined.
Warm the waffle iron and when it's ready, scoop our ¼ cup of the batter and place it in the center of the waffle iron. Close the lid and cook until the light turns green. Remove from the waffle iron with a fork.
Repeat with the remaining batter to finish the batch; you may need to add a bit of milk if the batter becomes very thick. Waffles may be frozen or stored in the fridge for up to 5 days.
Notes
The flour - I suggest using whole wheat pastry flour, but you can interchangeably use whole wheat flour or spelt flour and the waffles will turn out just right. I often just grab a bag of whatever is on sale or available at my local store - sometimes it's whole wheat pastry flour (which has a slightly finer grind) and sometimes it's whole wheat flour. Spelt flour is another whole-grain flour that is wonderful for baking and I'll use it often as well.
Add milk - Whole grain flour absorbs more liquid than all-purpose flour. You can add a little more milk, 1 tablespoon at a time, if you find the batter is too thick.
What size? I like to make my waffles using a ¼ cup measure, and a ⅓ cup measure also works well.
Make dairy-free - You can easily make these waffles dairy-free by substituting non-dairy butter for the butter, your favorite non-dairy milk for the milk, and non-dairy yogurt for the yogurt.
Swap coconut oil - Often, I will substitute the butter with unrefined or refined coconut oil for a change. The waffles are just as buttery and tender.
Make egg-free - Substitute a flax egg for the egg in the batter.
Make gluten-free - Use whole grain gluten-free flour like oat or sorghum flour, or a 1:1 all-purpose mix, using 280 grams of whatever flour you choose.