Combine warm water, 1/4 teaspoon sugar and yeast in a small bowl. Let sit 5 minutes until yeast is foamy. Combine 1/4 cup sugar, salt, 1/4 cup butter and vanilla in large mixer bowl. Add warm milk and stir to mix. Add yeast mixture, egg and flour. Beat on medium speed until well blended. Spread into greased 13 x 9-inch pan. Brush with 2 tablespoons melted butter. Cover and let rise in draft-free area until doubled, about 1 hour.
For Rhubarb Filling, combine rhubarb and water in large saucepan and cook over medium-high heat until the rhubarb is soft and beginning to lose its shape. Stir together corn starch, sugar, gelatin powder and cinnamon; add to rhubarb mixture. Continue to cook and stir until rhubarb is thick, 2 to 3 minutes. Remove from heat and cool.
To make Streusel Topping combine all ingredients in a bowl with a pastry blender until butter mixture is the size of peas.
Before baking cake, gently make small wells in the risen coffee cake, about 1 inch apart. Spoon about 1 teaspoon of rhubarb sauce into each well. Spread the remainder of the rhubarb filling evenly over the top of dough. Top with streusel topping.
Bake in a preheated 350°F oven for 30 to 35 minutes until the top begins to brown. Cool on wire rack for at least 15 minutes.
Combine all glaze ingredients together and drizzle over coffee cake. Best served warm.