Preheat oven to 350 degrees and set aside a muffin tin lined with paper muffin cups.
FOR CRUMBLE TOPPING, add shortening, flour, cocoa and salt to a small mixing bowl.
Using a fork, mash the dry ingredients into the shortening until an even crumble forms.
Set aside.
FOR CHOCOLATE RHUBARB MUFFIN BATTER, add flour, sugar, cocoa powder, baking soda and salt to a medium size mixing bowl.
Stir or whisk the dry ingredients together, removing any lumps and achieving an even color.
Add warm coffee, vanilla, vegetable oil and vinegar to the dry ingredients.
Stir until well blended.
Add chopped rhubarb and chocolate chips.
Stir until the rhubarb and chips are evenly distributed in the batter.
Scoop the batter into the paper muffin cups lining your muffin tin.
Retrieve the crumble mixture you set aside and sprinkle it evenly over the tops of all the muffins.
Place the filled muffin tin into a preheated 350 degree oven. Bake for 15-20 minutes depending on your oven. Check with a toothpick - if it comes out clean - the muffins are done.
Remove the muffin tin from the oven and transfer to a baking rack. When cool enough to handle, remove the muffin cups from the tin and place them on the baking rack to cool completely.
While the muffins are cooling, prep the LEMON ZEST GLAZE. You'll start by melting the butter in a small saucepan.
Stir in the powdered sugar. It will lump up. Add one teaspoon of water at a time until you reach a glaze consistency. A tiny amount of water goes a long way in thinning out this glaze. Keep the glaze warm, but do not let it bubble. When glaze feels like it is thin enough to drizzle onto muffins from a spoon, stir in the lemon zest.
Remove the saucepan from the heat and drizzle the glaze over the baked muffin tops. Work quickly, as the glaze will begin to solidify once off the heat.
