In the bowl of your stand mixer with dough hook attached, add milk, water, yeast and sugar. Let sit 10 minutes to allow yeast activation.
In another bowl, combine flour, salt, baking powder.
Add the flour mixture, sour cream and olive oil to the mixer bowl. Begin to knead dough over medium speed. Continue for about 10 minutes until dough forms into a soft ball around the hook.
Place ball of dough in a lightly greased bowl and cover with towel. Let rise for about 1 hour or until dough doubles in size.
Transfer dough onto a lightly floured surface and cut the dough into 4 equal pieces. Using a rolling pin, roll each piece out into a long rectangle.
Brush each flatbread with butter on both sides and preheat your grill to medium-low heat.
Place the flatbread on the grill and close lid. Let cook for about 2-3 minutes until the flatbread slightly bubbles on top and forms some nice grill marks. Flip flatbread over with a large spatula and let cook for another 2-3 minutes.
Remove from the grill and serve with your favorite toppings!