Preheat the oven to 350°F or 180°C.
Line a 9x13 baking dish with parchment paper. Allow it to be long enough to hang slightly over the edges. It will make it easier to take the brownies out of the dish after baking. In a medium size pot, melt the butter, dark chocolate, sugars, and salt over low heat. Stir frequently to avoid burning. Take it off the heat when everything is melted completely. Let it cool for 15 minutes.
Then add the eggs and vanilla extract, and mix until fully incorporated (a hand mixer works best). You can do that directly in the pan or pour the mixture into a large mixing bowl (or stand mixer bowl). Sift the flour, cocoa powder, and baking powder into the chocolate mixture, then add the espresso powder. Mix until just combined.
Add the chopped pecans and stir them in until they are evenly distributed.
Pour the batter into the lined baking dish, and with a scraper, smooth it out to have the same amount of batter across the baking dish. It helps the brownies to bake evenly.
Bake for 20-25 minutes for a more fudgy brownie; bake it a bit longer if you prefer a cake-like brownie.
Let the brownies cool for about 30-45 minutes before lifting them out of the baking dish, using the parchment paper overhang. Let them cool completely before assembling the Trifles.
Cut them into small squares (to fit the size of the glass jars you are using for assembling).
Since this is a big batch, you will likely have leftover brownies. Store the rest of the brownies in an airtight container or freeze them in a freezer container for up to two months.