Rhubarb Butter Cake or Gâteau Breton à la Rhubarbe

Brittany is known for its butter so it’s no wonder that there’s a cake which includes a lot of it. The original recipe had an apricot filling, but since I have an abundance of rhubarb, I used it for the filling instead.

Rhubarb Filling:
1 lb rhubarb, washed and cut into 1/4-inch dice
3/4 cup (6 oz / 170 g) granulated sugar
1 tablespoon vanilla sugar, optional
1 1/2 teaspoons fresh lemon juice
1 tablespoon water

Cake:
6 large egg yolks, divided
2 cups (9 oz / 255 g) unbleached all-purpose flour
1/4 cup (1 oz / 25 g) almond flour
½ teaspoon kosher salt
16 tablespoons (8 oz / g) unsalted butter, softened
¾ cup plus 2 tablespoons (6 1/4 oz /180 g) granulated sugar
2 tablespoons grand Marnier (or other orange liqueur)
Sliced almonds for garnish

This cake is traditionally served plain with coffee or tea but can be dressed up with fresh berries and maybe a dollop of crème fraîche if desired.

FOR THE RHUBARB FILLING:
Cook rhubarb, granulated sugar, vanilla sugar (if using), lemon juice, and water in a large heavy saucepan over moderately high heat, stirring pretty much constantly with a heat-proof spatula, until rhubarb exudes juices. Reduce heat and simmer while continuing to stir. Press the pieces of rhubarb to make a sauce and reduce to about 1 cup. This will take about 15 minutes. Remove to a shallow bowl or plate and cool to room temperature.

FOR THE CAKE:
Grease 9-inch round cake pan. Line with a round of parchment paper. Beat one egg yolk with a teaspoon of water and set aside.

Set a sieve over a medium bowl and sift together flours and salt. Set aside. In the bowl of a stand mixer using paddle attachment, beat butter on medium-high speed until smooth and lightened in color, 1 to 2 minutes. Add sugar and continue beat until pale and fluffy, about 3 minutes longer.

Add 5 egg yolks, one at a time, and beat until each are incorporated before adding the next one. Scrape down bowl, add orange liqueur and mix until combined, about another 1 minute. Reduce speed to low, add sifted flours and mix until just incorporated, about 30 seconds. Give batter final stir by hand.

Spoon half (about 13.5 oz / 390 g) of batter into bottom of prepared pan. Using small offset spatula, spread batter into even layer. Freeze for 10 minutes.

Spread cooled filling in even layer over chilled batter, leaving 3/4-inch border around edge. Freeze for 10 minutes. While the cake is freezing move a rack to the bottom third of the oven and set to fan-assisted 350 degree F.

Using offset spatula, carefully (but quickly) smooth the remaining batter over the entire surface. Brush with egg yolk wash. Using tines of fork, make light scores in surface of cake, spaced about 1 1/2 inches apart, in diamond pattern.

Bake until top is golden brown and edges of cake start to pull away from sides of pan, 45 to 50 minutes. Let cake cool in pan on wire rack for 30 minutes.

Run paring knife between cake and sides of pan, remove cake from pan, and let cool completely on rack, about 1 hour. Cut into wedges and serve.