Combine flour, sugar, baking powder, baking soda, and salt in a large bowl.
In a separate bowl, beat buttermilk, milk, eggs, melted butter, and vanilla.
Using a wooden spoon, mix the wet and dry ingredients until just combined. Don't overmix! There will probably be flour lumps, which is okay.
Lightly oil a Poffertjes griddle with non-stick cooking spray. Pour in 1 teaspoon of batter into each spot and cook until bubbles form on the top, then use a skewer or fork to flip to the other side.
Cook until golden brown, about 1-2 minutes per side. (Spray the griddle with cooking spray in between batches.)
Remove from the griddle and sprinkle with powdered sugar, berries, syrup, and any other favorite toppings.
Video
Notes
Store cooled poffertjes in an airtight container in the refrigerator for 2-3 days or in the freezer for up to 3 months.