Lemon Rhubarb Mascarpone Mousse Cake
Reipe from Tartelette
For the tulip paste:
4 ounces unsalted butter, softened
4 ounces powdered sugar, sifted
1/2 cup egg whites, at room temperature
4 ounces all purpose flour, sifted
1 1/2 Tb cocoa powder.

Cream the butter and powdered sugar together in a stand mixer fitted with the paddle attachment. Sift the flour and cocoa powder together and add to the butter mixture and mix until incorporated. Do not overmix. Use right away or store in the refrigerator, taking it out 30 minutes before you plan to use it.
Pour the batter into a piping bag fitted with a small plain tip. Line a baking sheet with parchment paper. Using a ruler set on top of the sheet as your guide, pipe lines with the tulip paste onto the parchment paper, or any shape you desire. You can also smooth a large amount of the paste on the paper and using a ruler set on the rim of the pan, use a decorator comb to drag on the batter to form lines. When I make somewhat straight lines, this is the method I use, and I use the pastry bag method for lace patterns and others. Set the baking sheet in the freezer until the tulip paste is firm. Keep frozen until you are ready to pour the cake batter on top.

For the ribbon sheet cake:
4 ounces finely ground almonds (I used slivered)
3 ounces powdered sugar
1 ounce all purpose flour
4 eggs
1 egg yolk
3 egg whites
1 ounce granulated sugar
1 ounce melted unsalted butter

In the bowl of a stand mixer fitted with the paddle attachment, beat the almonds, powdered sugar, flour and 2 eggs until combined. Scrape the sides and bottom of your bowl with a spatula and add the remaining 2 eggs and the yolk until well combined. Set aside.
Whip the egg whites in a separate bowl until they appear foamy, add the granulated sugar in a steady stream and whip until you get a glossy meringue. Fold the meringue mixture into the egg/flour mixture. Stir in the melted butter and fold until all the ingredients are well incorporated but without deflating your meringue base.

Remove the baking sheet with the tulip paste from the freezer and pour the cake batter on top. Bake at 450F for about 6-8 minutes or until the sponge becomes to color. Remove from the oven, let cool a few minutes and invert your cake onto a piece of parchment paper. Peel the baking paper. Let cool before using. You can keep it wrapped in the refrigerator for up to a week or in the freezer for a month. Bring it back to room temperature before you fit it in the pastry rings. Line 6-8 pastry rings (depending on their size, mine are 3 inches wide) with parchment paper, cut 1.5 inches wide strips of cake and cut them to fit around the inside of your rings. Divide the mousse inside the rings and let set.

For the mousse:
1 egg, separated
2 TB sugar
2 oz mascarpone cheese, at room temperature
75 ml. heavy cream
1 tsp. powdered gelatin (1/2 sheet) + 1 Tb water
zest and juice of one lemon
1 cup cooked rhubarb

Sprinkle the gelatin over the water, stir and let sit to bloom. In a large mixing bowl, whisk the mascarpone with the sugar, add 1 egg yolk and whisk until well incorporated. Heat the gelatin for about 10 seconds in the microwave and quickly whisk it in the mascarpone batter. Add the lemon juice and zest. Whip the egg white until stiff, fold into the mascarpone mixture. Add the cooked rhubarb. Whip the heavy cream to medium stiff peaks, and fold into the mascarpone. Divide it evenly and carefully among the pastry rings. Store in the refrigerator for 2-4 hours or overnight.