Beat the room-temperature butter with a hand mixer for about 1 minute, until fluffy. Add the sugar and beat everything together for a few minutes, until light and fluffy.

Add the grated marzipan in 4–5 additions, mixing it in with the hand mixer as evenly as possible. At first, it will be harder to get smooth, but by the end, once you’ve added all the marzipan, it will come together nicely.

Now add the eggs one at a time, mixing everything completely after each addition.

In a bowl, mix together the flour, baking powder, and salt, then fold it into the marzipan mixture with a spatula. Do not overmix.

Line the bottom of an 8- to 9-inch (20–22 cm) cake pan with parchment paper, and generously butter the sides, roughly halfway up.

Pour the batter into the pan and smooth it out as much as possible with a spatula.

Bake for about 40 minutes in the oven preheated to 350°F (180°C). When the cake is done, take it out of the oven and let it cool completely in the pan. Baking time may vary, so it’s worth checking the cake after 35 minutes. The top will be golden brown, and it’s ready when the whole surface is lightly springy and the center no longer looks wobbly. Do not overbake it.

Remove the cake base from the pan and place it on the large plate or cake stand you’ll serve it on.

Chop the chocolate with a sharp knife.

Melt the butter in a saucepan, then turn off the heat. Add the chocolate and stir with a spatula until it has completely melted and is beautifully fluid and smooth. You may need to put the saucepan back on the heat a few times during this step. If you’re using gas or induction, put it back over the lowest flame or setting; if you’re using an electric cooktop, use only the residual heat. Put it back on occasionally for 5–10 seconds at a time, stirring constantly.

Pour all of the chocolate sauce into the center of the cake. Use a spatula to smooth it outward toward the edge. Let it run down over the edge and coat the sides as well. Mazarin cake is rustic, so the coating does not need to be perfectly smooth. Coat the entire surface, including the sides, because this is what keeps it from drying out.

Let the chocolate glaze set completely at room temperature. Store the cake covered at room temperature. It’s delicious served at room temperature, too, but the flavors come through even more if you put it in the fridge for 30–60 minutes before serving.