Preheat the oven to 350ºF. Line a 6-inch cake tin with parchment long enough to cover up the sides to the top of the pan.
Add the rhubarb to a small bowl, and toss with 3 tablespoons (35 grams) of sugar.
In a separate small bowl, add the butter and beat until creamy, 30 seconds. Add the sugar, and beat until light and fluffy, 2 to 3 minutes.
Add the egg, lemon zest, and vanilla, Beat until fluffy and well mixed, 1 minute.
Add the flour, baking powder, salt, and milk. Beat on low until just mixed, then increase speed to high for 15 seconds.
Scrape the batter into the prepared pan. Evenly spread the rhubarb on top. It'll seem like too much—don't worry. The batter rises over it as it bakes, enveloping the rhubarb.
Bake for 1 hour, until a skewer inserted in the center comes out clean of batter (there might be some sticky rhubarb-like jam on it). Let the cake cool completely before cutting, preferably all day, or overnight.