For the cake:
Position a rack in the middle of the oven and preheat the oven to 350°F. Lightly butter the bottom and sides of a 17 by 12 by 1-inch baking sheet (half sheet pan). Line the bottom with parchment, butter the parchment, then flour the parchment and the sides of the pan. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, mix the butter and almond paste on medium speed, scraping down the sides as necessary, for 3 to 5 minutes, until the mixture is light and creamy. Add the sugar and cocoa and blend on low speed. Increase the speed to medium and add the eggs one at a time, beating after each addition until incorporated. Continue to mix for 2 to 3 minutes, or until lightened, scraping down the sides as necessary.
Pour the batter into the prepared pan and spread it evenly, making certain to get in the corners. Bake for 25 minutes, or until just set: the surface should still be slightly spongy to the touch, but a skewer inserted in the center will come out clean, and the cake will begin to pull from the sides of the pan.
Remove from the oven and cool on a cooling rack for 5 to 10 minutes. Turn out onto the rack and cool completely.
Place the cake in the refrigerator for 20 minutes or the freezer for 10 to 15 minutes so it will be easier to cut.
To fill the cakes:
Using a 2½-inch round cutter, cut 24 disks from the cake. Brush the top of 12 of the disks with the melted chocolate, and top with the remaining 12 disks. Place a cooling rack over a baking sheet. Arrange the cakes on the rack, leaving ample room between them. Let the cakes come to room temperature before glazing.
To glaze the cakes:
Scatter the butter over the bottom of a medium skillet and top with the chopped chocolate. Place over medium heat and let sit until about three-quarters of the chocolate is melted. Remove the pan from the heat and whisk just to combine and melt the remaining chocolate; whisk gently, to avoid creating bubbles. Transfer the glaze to a measuring cup with a spout. Slowly pour some glaze over the center of one cake, then use a small offset spatula to push the glaze over the sides, coating the top and sides of the cake. Smooth the sides to cover if desired. Repeat with the remaining cakes. If you use all the glaze, remove the rack from the pan, remove any crumbs from the glaze that has dripped into the baking sheet, and, if necessary, reheat the glaze before using it.
Let the cakes sit at room temperature until the glaze sets, then transfer to a platter or individual plates.