1. Preheat oven to 350-365°F (175-185°C). Line baking sheets with parchment paper.
2. In a stand mixer, cream together softened butter, brown sugar, and granulated sugar until fluffy (1-2 minutes). Add eggs, light corn syrup, vanilla extract, (and dissolved espresso, if using), mixing until combined.
3. In a separate bowl, whisk flour, cocoa powder, cornstarch, baking soda (or baking powder), and salt. Gradually add dry ingredients to wet, mixing just until incorporated.
4. Portion dough into ¼ cup (about 60 g) balls, place 6 per sheet spaced apart. Flatten gently with palm or bottom of a scoop into thick disks (~½ inch thick).
5. Bake 8-11 minutes until edges are set with slight cracks but centers remain soft and not shiny. Cool on sheet 5 minutes, then transfer to wire rack to cool completely.
6. For meringue frosting: Heat sugar and egg whites in a heatproof bowl over simmering water until sugar dissolves and temperature reaches 160°F (71°C). Transfer to stand mixer and whip on medium until stiff peaks form (~4-5 minutes). Fold in marshmallow fluff and vanilla, whipping again until stiff, glossy peaks form. Optionally add butter and cream for a lighter texture. Pipe frosting onto cooled cookies in a swirl starting from the center outward.
7. To make ganache: Heat heavy cream (and corn syrup, if using) to simmering, pour over chopped chocolate, let sit 2-5 minutes. Whisk from center outward until smooth and glossy. Let thicken slightly before use. Either pipe a dollop onto the marshmallow frosting or dip the frosting-topped side into the ganache and set on a rack to let ganache firm (about 15-30 minutes).
8. Melt white chocolate wafers or chips in 30-second intervals in the microwave, stirring until smooth. Transfer to piping bag or a plastic zip-top bag with a small corner snipped. Pipe fine loops or squiggles atop each cookie. Let white chocolate set completely before serving.