1/2 cup butter
1 cup white sugar
1 3/4 cups all-purpose flour
1 teaspoon baking powder
1 teaspoon baking soda
4 eggs
3 1/2 cups blueberries, divided
1 cup miniature marshmallows
1 cup brown sugar
1/2 cup butter
1 teaspoon vanilla extract
1/4 cup water
3 tablespoons cornstarch
Preheat the oven to 375 degrees F (190 degrees C), grease cookie sheets.
In a large bowl, cream together the sugar, sifted together with 3/4 cup flour. Beat in the eggs, then beat in the blueberries, marshmallows, brown sugar and butter. Combine the 1/2 cup flour and baking powder; stir into the creamed mixture until just blended. Fold in the vanilla and water. Drop by rounded spoonfuls onto the cookie sheet.
Bake for 30 to 35 minutes in the preheated oven, or until slightly brown on the edges. Allow cookies to cool on baking sheet for 1 minute, before removing to a wire rack to cool completely.