Preheat oven to 350°F (175°C).
Line a small pan
Bloom the coffee
Mix: instant coffee + hot water. Set aside.
Melt butter in a bowl (microwave or stovetop). Let cool slightly (warm, not hot).
Mix wet ingredients
Whisk together: Melted butter and sugar
Then whisk in:
egg + egg yolk
vanilla
coffee mixture
Mix until smooth and slightly glossy.
Add dry ingredients
Whisk together: flour, cocoa, salt
Add to wet ingredients and mix just until combined.
Pour into prepared pan.
Bake 18-22 minutes until: edges are set and center is slightly soft (do not overbake).
Cool