Put the baking pan in the oven and preheat to 180°C (350 F). We need a hot pan to welcome the batter.In a bowl sift the flour, add the water and mix using your hand or a kitchen whisk. Add the eggs separately and whisk until you have a smooth batter with no lumps of flour.Brush your hot baking pan (be very careful) with generous amount of olive oil and pour the batter into the baking pan.Scatter the pieces of feta cheese and finally the butter cubes all over.Place the baking pan in the middle oven rack and bake for about 40-50 minutes, until golden brown.