Preheat oven to 350°F and line an 8×8 pan with parchment.
Melt vegan butter and pour into a bowl.
Whisk in sugars until paste forms.
Mix in eggs and vanilla.
Sift in cocoa, salt, flour; whisk until just combined.
Spread into pan and smooth.
Bake 30–40 min, check at 30.
Cool fully before slicing.
