1. Preheat the oven to 350 degrees. Butter the bottom and sides of the pan and line the baking pan with parchment paper leaving approximately a 2-inch overhang on all four sides of the pan.

Make the crumble topping

1. Mix the butter, brown sugar, cinnamon, and salt and then add the flour and mix again until large clumps have formed. Place in refrigerator until you are ready to use it.

Make the cake

1. In one medium bowl, mix the rhubarb, brown sugar, and 1/4 cup of flour. In a small bowl combine 3/4 cup of flour, baking soda, and salt. In the large bowl of an electric mixer, beat the butter and confectioner's sugar until it becomes fluffy and light. Beat in the eggs one at a time and then add the vanilla extract and flour mixture and beat until combined. Pour the batter into the square pan and spread the rhubarb mixture on top of the cake. Sprinkle the crumb topping evenly over the rhubarb mixture.
2. Bake until golden brown on top, about 40 to 50 minutes.
3. Remove from the oven, place on a wire rack, and let them cool to room temperature. Once cooled, using the parchment paper overhangs, lift the bars out of the pan and place them on a cutting board. Cut the cake into 9 pieces.
4. While the crumb bars are baking, make the cream. Vigorously whisk the egg yolks and sugar in a medium saucepan and stir in the cream. Whisk again and continue cooking over low heat until the mixture thickens about 15 to 20 minutes. Remove from the heat and stir in the vanilla extract.

For serving

1. Place a bar in a small bowl and drizzle the vanilla cream over the top.