How to Make It
Make the cake Preheat the oven to 375°. Brush a 9-by- 4-inch loaf pan with butter. In a medium bowl, whisk both flours with the baking powder and salt. In a small bowl, mix the 2 tablespoons of melted butter with the honey and vanilla seeds.
In a stand mixer fitted with the whisk, beat the egg yolks with 1/2 cup of the granulated sugar at high speed until thick and pale, about 5 minutes. Scrape into a medium bowl. Clean the mixer bowl and whisk.
In the stand mixer fitted with the whisk, beat the egg whites at high speed until soft peaks form, about 2 minutes. With the machine on high, gradually add the remaining 1/2 cup of granulated sugar and beat until stiff, about 3 minutes more. Using a rubber spatula, fold a large scoop of the beaten egg whites into the egg yolk mixture to lighten it. Gently fold the egg yolk mixture into the egg whites until no streaks remain.
Sift the dry ingredients over the eggs, then gently fold them in. Fold in the butter mixture. Scrape the batter into the prepared pan and smooth the top. Bake for about 25 minutes, until springy and browned. Let cool completely, then unmold onto a platter and dust with confectioners’ sugar.
Meanwhile, make the jam In a large mortar, using a pestle, lightly crush the blackberries with the granulated sugar, tarragon, gin, lemon juice and pepper until a coarse jam forms. Transfer to a serving bowl. Cut the cake into slices and serve with the jam.