Step 1

Heat the oven and prep the pan: Arrange a rack in the middle of a regular or toaster oven and heat to 350°F. Place the butter in a mini square or round metal pie pan and place the pan on top of the toaster oven or inside the regular oven as it heats to melt. Pour the melted butter into a small heatproof bowl. Use a brush to coat the pan with the residual butter left in the pan; set aside.

Step 2

Mix the dry ingredients: Whisk together the flour, brown sugar, sugar, and cocoa powder in a small bowl until thoroughly combined.

Step 3

Add the wet ingredients: Add the melted butter, egg yolk, coffee or water, and vanilla. Stir with a rubber spatula to combine. Do not overmix.

Step 4

Bake the brownie: Pour the batter into the prepared pan. Top with chocolate chips or walnuts if using. Bake for 15 minutes for a gooey center or 20 minutes for a cakey brownie.

Step 5

Cool the brownie: Remove from the oven and cool for 5 minutes before enjoying. Serve with ice cream, if desired.