Fudgy Brownie Cake

1 Generously grease the inside of the slow cooker canister by holding a small slice of butter with a paper towel and rubbing the butter all over the surfaces. Add some flour, then turn the crock this way and that to coat both the bottom and the walls with a fine, even dusting. Tap out any excess flour.
2 Set up a double boiler with about 1 inch of water in the bottom pan, simmering over medium heat—or place a large, heat-safe metal bowl over a medium saucepan with a similar amount of simmering water. Add the butter and both chocolates. Stir with a rubber spatula until about two-thirds of the chocolate has melted. Remove the top half of the double boiler or the bowl from the saucepan below, then continue stirring off the heat until all the chocolate has melted. Cool for 10 minutes.
3 Meanwhile, beat the sugar, eggs, and vanilla in a large bowl with an electric mixer at medium speed until quite thick and pale yellow, up to 10 minutes.
4 Beat in the cooled chocolate mixture until smooth, then scrape down and remove the beaters. Fold in the flour and salt with a rubber spatula, using wide, gentle arcs to make sure every speck of flour has been moistened. Pour and spread this mixture into the slow cooker.
5 Lay long strips of paper towels across the top of the slow cooker, overlapping each other and hanging down the sides for stability. Set the lid in place, then cook on high for about 1 hour 20 minutes in a small (2- to 3 1/2-quart) slow cooker, about 1 hour 40 minutes in a medium (4- to 5 1/2-quart) one, or about 2 hours in a large (6- to 8-quart) model, or until the cake is firm to the touch and the edges are dry while the middle is dark and moist. Unplug, uncover, and cool in the canister for 30 minutes before cutting out slices with a nonstick-safe knife or setting a cutting board over the cooker, inverting it, and turning the cake out.