Fudgy Brownie Cake

Ingredients for a 2- to 3 1/2-quart slow cooker:
6 tablespoons (3/4 stick) unsalted butter, cut into small bits, plus more for greasing the canister
6 1/2 ounces dark chocolate, chopped
1 1/2 ounces unsweetened chocolate, chopped
3/4 cup sugar
1 whole large egg, plus 1 large egg yolk, at room temperature
1/2 tablespoon vanilla extract
1/2 cup all-purpose flour, plus more for coating the canister
1/4 teaspoon salt

Ingredients for a 6- to 8-quart slow cooker:
12 tablespoons (1 1/2 sticks) unsalted butter, cut into small bits, plus more for greasing the canister
13 ounces dark chocolate, chopped
3 ounces unsweetened chocolate, chopped
1 1/2 cups sugar
3 large eggs, at room temperature
1 tablespoon vanilla extract
1 cup all-purpose flour, plus more for coating the canister
1 teaspoon salt

TESTERS' NOTES
• Because of the way the slow cooker traps moisture inside, brownies come out fudgy, dense, and moist. The edges, however, will dry out a bit—and be a boon to those who like cakey brownies.
• When you're melting chocolate over a double boiler, control the heat so the water below barely simmers. Too much steam can condense into the chocolate, causing it to seize—that is, break into threads and a thin liquid. If the chocolate seizes, there may be no help for it. Try stirring in a little cream to see if the mixture will cohere. You may have to start over, even with the best of intentions.
• Room-temperature eggs stabilize batters, building structure in tiny air pockets that create that vaunted crumb, the hallmark of good baked fare. To bring eggs to room temperature, leave them on the counter (but still in their shells) for 20 minutes, or immerse them (again, still in their shells) in a bowl of warm (not hot) tap water for 3 to 4 minutes.
• We also like these with a little kick from up to 1/2 teaspoon cayenne added with the salt.

SHORTCUTS Omit greasing and flouring the slow cooker and instead use baking spray, a mix of oil and flour.

INGREDIENTS EXPLAINED Good-quality chocolate is most often sold with a percentage on the label—55 percent, 60 percent, 70 percent, and so on. The number refers to the percent of cocoa solids (the bitter chocolate stuff) as opposed to the sugar, cocoa butter, and certain stabilizers. As a general rule for this book, 40 to 55 percent is semisweet chocolate, 60 to 65 percent is bittersweet, and 70 to 85 percent is dark chocolate.
Milk chocolate is just that: chocolate with milk added to the mix. And unsweetened chocolate, sometimes called baking chocolate, has no (or in some brands, very little) sugar in the mix.

ALL-AMERICAN KNOW-HOW For the best baked goods, beat and beat and beat the sugar and eggs. The more air you add, the more loft you'll get. Yes, you can beat in so much air that the cake will not hold together when cut; but in general, beat until you think you've got a smooth, thick mixture—then beat it some more.
However, once you add the flour, stop beating altogether. Fold the flour mixture into most batters with a rubber spatula (yes, there are exceptions). Beating will elongate the flour's glutens, resulting in tough, chewy cakes. Folding will hold off said elongation. Don't overdo even the folding: work just until you see no more dry pockets. Some graininess from moistened but undissolved flour is expected—and encouraged.