Preheat the oven to 300 degrees F.

Grease and flour a 9'x9" pan.

In a separate bowl, whisk together the flour,  baking powder, and salt.

Cream together the butter, 1/2 cup of the sugar, and vanilla extract. Add the eggs one at a time, making sure each is fully incorporated into the batter before adding the next.

Add the dry ingredients and milk and mix until just combined.

Spread the batter out onto the prepared pan. Place in the oven and bake for 20 minutes.

Remove from the oven, let cool and then transfer the cake to a wire cooling rack.

Trim the ends off the rhubarb. Wash and pat dry. Cut the stalks into pieces.

Place the pieces in a saucepan with the water and 1/2 cup of the sugar and boil for 10 minutes.

Let the mixture cool completely.

Transfer the cake to a serving platter. Spread the rhubarb mixture on top of the cake layer

Place the cake in the refrigerator for one hour.

Beat together the cream cheese, vanilla bean paste and the last 1/4 cup of sugar in a mixer fitted with the whisk attachment.

Beat the heavy cream separately until stiff peaks form and fold it into the cream cheese mixture.

Remove the cake from the refrigerator and spread the cream layer on top of the cake.

Use a serrated knife, fork or decorating comb to comb the top of the cake, squiggly lines, creating a wave look.

Sprinkle the cocoa powder evenly over the top of the cake.

Let the cake stand one hour in the refrigerator before cutting and serving.