How to Make It
Preheat the oven to 375°. Pour 1/2 inch of water into an 8-inch square baking pan. Add the squash, cover with foil and roast for 1 hour, or until tender. Turn down the oven temperature to 350°. Drain the squash and puree it in a food processor; you should have 1 1/2 cups.
Lightly butter a 9-by-13-inch metal baking pan and line the bottom with parchment paper; butter the paper. Bring 1 inch of water to a gentle simmer in a small saucepan. Set a medium heatproof bowl over the saucepan and add the chocolate and butter. Let melt, stirring frequently. Set aside and let cool slightly.
In a large bowl, beat the eggs with the sugar, vanilla and salt at medium speed, until pale and thick, about 2 minutes. Beat in the melted chocolate and the squash. At low speed, beat in the flour and baking powder. Pour the batter into the prepared baking pan and bake for 25 minutes, or until a cake tester inserted into the center comes out clean. Transfer the pan to a wire rack and let the cake cool completely before cutting into squares.