mix the ingredients for the streusel topping together until it's crumbly, and set aside
beat the butter and both sugars on medium until it's fluffy
scrape down the sides of the bowl as needed
add the egg, and beat well, scrape the bowl again
stir in the vanilla
in a separate bowl stir together the flour, soda, and salt
add it to the butter in the mixing bowl
mix until its combined
then stir the rhubarb in
divide into 10 cookies and place on a baking sheet about 2 inches apart
pat down a little bit
divide the streusel topping between the cookies and press in lightly
chill for 15-20 minutes
preheat the oven to 350°
bake the cookies for 12-13 minutes until the edges are just lightly browned
leave to cool on the baking sheet for 5 minutes
them gently remove to a baking rack to cool
when the cookies are cool put the powdered sugar in a small bowl drizzle in the vanilla and 1 TBSP of heavy cream and stir to combine, add just enough of the remaining cream JUST until you can drizzle the icing, the thinner you make it the quicker it will drip off the sides of you cookies so make it JUST thick enough to drip or drizzle
drizzle over the cookies and let the icing harden up