Brew 1 1/4 cups (300 ml) of strong espresso or coffee and let it cool to room temperature. Stir in 1 tsp instant espresso powder, if using, until it fully dissolves for a deeper coffee flavor. Add 1 tbsp coffee liqueur or dark rum if you want it, then set the bowl aside.
In a large bowl, beat the 8 oz (225 g) softened mascarpone on low with the hand mixer for about 30 seconds, just until smooth. Add the 2 cups (480 g) Greek yogurt, 1/3 cup (110 g) honey, 2 tsp vanilla, and the pinch of salt. Beat on low to medium only until thick and creamy, about 1 minute; keep the speed down so the filling does not loosen.
Dip each ladyfinger into the coffee for about 1 second per side, then lay it flat in the 8-inch dish. Use about 12 ladyfingers to cover the bottom in a single, snug layer. Spread half of the filling evenly over the soaked ladyfingers with the spatula.
Dip and arrange the remaining 12 ladyfingers over the filling in a second single layer. Spread the rest of the filling on top and smooth it flat.
Cover the dish and refrigerate for at least 4 hours, ideally overnight. This lets the layers firm up and the coffee and cream flavors settle together.
Just before serving, dust the top evenly with 2 tbsp cocoa powder through the fine-mesh sieve. Shave the 1 oz (30 g) dark chocolate over the top if using, then slice into 9 squares.