Preheat oven to 375 degrees. Line rhubarb in the bottom of an 8x9-inch non-reactive pan. In a bowl, mix 3/4 cup sugar, butter, baking powder, salt, vanilla, almond extract, milk, and flour. Pour over rhubarb. Mix together remaining sugar and cornstarch. Sprinkle over mixture in pan. Pour boiling water over the top. Bake for 45 minutes.