Breton Buckwheat Cake with Fleur de Sel

INGREDIENTS

• For the cake:
• 7/8 cup (140g) buckwheat flour
• 1 cup (140g) all-purpose flour
• 1/2 teaspoon plus 1/3 teaspoon fleur de sel
• 1/4 teaspoon ground cinnamon
• 1/2 pound (240g) unsalted butter, at room temp
• 1 cup (200g) sugar
• 4 large egg yolks
• 1 large egg
• 3/4 teaspoon vanilla extract
• 2 tablespoons dark rum
• For the glaze
• 1 large egg yolk
• 1 teaspoon milk