Preheat oven to 350 degrees.
Spray a 7-inch tart pan with removable bottom with cooking spray. If you don't have a tart pan, use a 6- to 7-inch springform pan. Line bottom of pan with parchment paper and spray the paper.
Melt 6 ounces chopped chocolate and butter in a heavy saucepan over medium-low heat. Stir until chocolate and butter are melted and smooth.
Add sugar and salt and reduce heat to low. Cook while stirring for about one minute, until sugar starts to dissolve.
Remove pan from heat. Whisk in eggs, one at a time. The mixture will look smooth and glossy.
Use a mesh hand sieve to sift cocoa into mixture -- this prevents lumps. Whisk until cake batter is smooth.
Pour batter into prepared pan and bake in preheated oven for about 25 minutes. The center of the cake should be just firm to the touch, but be careful to not overbake cake or it will be dry.
Cool in the pan on a wire rack for 15 minutes. Invert the cake on a plate.
To prepare glaze, melt 2 ounces chopped chocolate and 1 1/2 Tbsp unsalted butter in same saucepan you used to make cake. When melted and smooth, remove pan from heat.
Add milk and agave syrup or honey and vanilla and stir until smooth and glossy. Let glaze cool for about 3 minutes before pouring it on cake.
Pour all glaze in the middle of the cake. Use a silicone or rubber spatula to spread glaze evenly over cake, allowing it to evenly run down the sides of the cake.