Preheat oven to 325. Line the bottom of a 9 inch springform pan with parchment paper. Grease the paper and sides of the pan with butter. Wrap the bottom of the pan with two layers of aluminum foil and place on a baking sheet.

Mix together the rhubarb, cornstarch and ½ c of sugar. Place in the bottom of the greased pan.

Sift together flour, baking powder and salt

Cream together 2 sticks of butter for 2 minutes. Using your fingers, combine the lemon zest and remaining cup of sugar, rubbing together til uniform in color. Add sugar mixture to the creamed butter and continue to mix for 5 minutes.

Meanwhile, heat brown sugar and half a stick of butter in a small saucepan over medium heat, stirring constantly until smooth and bubbly. Pour over the top of the rhubarb.

Add remaining wet ingredients to creamed butter mixing in one ingredient at a time until fully incorporated.

Add the dry ingredients mixing in a ¼ c at a time and scraping down the sides as needed. Do not over mix. The batter will be thick.

Spread on top of the rhubarb and smooth out the top.

Place in over on center rack and bake for about 1 hour and 15 minutes. Cake is ready when a knife comes out clean.

Cool on wire rack for 15 minutes. Then, slide a knife around the edge of the pan. Turn the cake out onto a serving plate to finish cooling.