Whisk together 1 cup flour, sugar, baking powder, cinnamon and salt. In another bowl, whisk together egg, milk and melted butter; add to dry ingredients, stirring just until moistened (batter will be thick). Spread onto bottom of a greased 5-qt. slow cooker.
Combine peaches, the remaining 1/4 cup flour, brown sugar, lemon juice and vanilla; spoon over batter. Cook, covered, on high until peach mixture is bubbly, 1-3/4 to 2 hours. If desired, serve with vanilla ice cream.