Preheat the oven to 350°F (175°C).

Make the apples: Heat a medium (10-inch, 25cm) skillet over medium-high heat. Add the butter, apples, and sugar and stir well until the apples are well coated. Cook, stirring or tossing occasionally, for 10 to 12 minutes, or until soft and caramelized on all sides. If the apples start browning too much, reduce the heat to medium. Set aside on a plate to cool while you make the batter.

Make the cake: In a medium bowl, combine the buckwheat flour, all-purpose flour, baking powder, and sea salt. Whisk to combine thoroughly. Set aside.

In the bowl of a stand mixer fitted with the paddle, cream the butter and sugar for 2 to 3 minutes, or until pale and fluffy, scraping once or twice. Add the egg yolks, one at a time, beating well after each addition and scraping as needed. Add the vanilla and brandy and beat again until mixed.

With the mixer off, add the dry ingredients, then mix on low until the flour is moistened. Turn up to medium and beat until well mixed.

Butter a 9-inch (23cm) springform pan. Spread half the batter in the bottom of the pan, using a small offset spatula to smooth it. Add the apples in an even layer. Add the remaining batter and use damp hands to spread it out over the apples. Do a final smoothing with the spatula, but it’s ok if it’s a little lumpy.

Place on a rimmed baking sheet lined with parchment and bake for 45 to 50 minutes, or until the cake is golden brown on top and feels firm in the center. Let cool on a wire rack for about 15 minutes before removing the ring.

Finish the cake: Melt the butter in a small skillet and add the buckwheat groats. Cook, stirring, over medium-high heat for 5 to 7 minutes, or until a shade darker. Transfer to a small plate to cool.

Warm the apricot preserves in a small pan or the microwave until loose. Push it through a small sieve. Discard the solids. Spread the strained preserves evenly over the top of the cake and sprinkle with the buckwheat groats. Let cool completely, or enjoy it slightly warm.