Preheat the oven (with its rack positioned in the center) to 425°F. Butter and line a 9-by-13-inch cake pan with aluminum foil, shiny side down, then butter the foil as well. Set the prepared pan aside.
To the top of a large double boiler or to a medium metal bowl placed over a medium pot of simmering water, add the remaining butter and chocolate, and warm, stirring occasionally, until completely melted. Turn off the heat and set aside.
d with the whip attachment, add the eggs, sugar, instant espresso, vanilla and almond extracts, and salt, and beat on high speed until very thick and voluminous, 10–12 minutes. Adjust the speed to low and add the chocolate mixture (which may still be warm) and beat only until mixed, scraping down the sides and bottom of the bowl as needed to combine. Add the flour and again beat on low speed only until mixed. Remove the bowl from the mixer and use a silicone spatula to fold in the walnuts.
Pour half of the mixture (about 3½ cups) into the prepared pan and use the spatula to smooth it into an even layer. Place a layer of the mints, touching each other and the edges of the pan, evenly over the batter. Cut some mints to fill in any large spaces along the edges. (You might not need all of the mints.) Pour the remaining batter over the mints and use the spatula or the back of a spoon to smooth the top. Transfer to the oven and bake until the brownies have a firm crust on top, but a toothpick inserted in the center still comes out wet and chocolatey, 33–35 minutes, rotating the pan 180° halfway through baking. Remove from the oven and set aside to cool to room temperature.
Cover the pan with a large baking sheet and invert the pan and the sheet. Remove the baking pan and foil lining, then cover the brownies with a length of wax paper and another baking sheet and invert again, leaving the slab of brownies right side up. Refrigerate for at least three hours or overnight. (Do not attempt to cut them before chilling!)
When you are ready to cut the brownies, use a long, heavy knife with a sharp blade (either serrated or straight) to cut the slab into 32 bars. Serve immediately, pack in an airtight box, or wrap individually in cellophane, wax paper, or foil. These freeze perfectly and can be served very cold or at room temperature.