Preheat the oven to 350º F. Butter and flour a 9-inch square baking pan. In a microwave oven or double boiler, melt half of the chocolate and the 2 sticks butter; stir to combine. Set aside.
In a large bowl, using a whisk, beat the sugar with the eggs until bubbly, about 1 minute. Gradually whisk in the melted chocolate, then the flour. Using a spatula, scrape the batter into the prepared pan, smooth the top and bake until a toothpick inserted in the center comes out with moist crumbs, about 35 minutes. Let cool on a rack.
In a microwave oven or double boiler, melt the remaining chocolate and 1 1/2 tablespoons butter; stir to combine. Pour the glaze onto the cooled truffle cake and tilt the pan to coat the top. Refrigerate to partially set the glaze, about 30 minutes. While the chocolate is still tacky, scatter the raspberries in a single layer on top, then refrigerate for a few more minutes to set the glaze completely. Cut the cake into 3-inch squares.