Preheat oven to 350 degrees F.
Grease a 13 x 9 baking pan, set aside.
Prepare the filling by combining rhubarb and berries together in a large heavy saucepan. Add lemon juice and zest.
Cook over medium heat for about 5 minutes or until rhubarb has softened but not lost it's shape.
Combine sugar and cornstarch together in a small bowl, stir well.
Add sugar/cornstarch mix to the saucepan.
Bring to a boil, cook, stirring constantly for 2 minutes until thickened.
Remove from heat and set aside to cool while you make the cake.
In the bowl of your food processor, pulse 1 box of Savannah Smiles Girl Scout Cookies until they resemble coarse sand
Measure out 1/2 cup of cookie crumbs and put in a large bowl along with the flour, sugar, baking powder and salt.
With two forks, or a pastry cutter, cut butter into flour/cookie mixture until you have coarse crumbs
In a medium bowl mix together milk, lemon juice, zest and eggs.
Stir wet mixture into dry flour/butter mix.
Spread half the batter into prepared 13 x 9 baking pan
Gently spread cooled rhubarb/berry filling on the top of the batter, going almost to the edges of the pan, but leaving about a 1" border around the edges
Carefully spread remaining batter over the jam layer.
Before putting in the oven, make the topping.
In a microwave safe bowl, melt butter.
Stir in remainder (approx 1 1/2 cups) Savannah Smiles Cookie crumbs into butter and stir well until all the crumbs are coated and the mixture looks like wet sand.
Sprinkle topping over coffee cake batter
Bake coffee cake on the middle rack of your oven for 40 - 45 minutes.
If topping starts to brown too much, lightly cover the cake with a sheet of foil, but do not crimp down the edges
Cool on a rack for 30 minutes before cutting into squares