Preheat your oven to 350°F (175°C). Grease and flour an 8-inch round cake pan.
In a large mixing bowl, combine the flour, granulated sugar, cocoa powder, baking powder, and salt. Mix well.
In another bowl, whisk together the melted butter, eggs, and vanilla extract until smooth.
Gradually add the wet ingredients to the dry ingredients, stirring until just combined. Do not overmix.
Pour the batter into the prepared cake pan and smooth the top. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cake to cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.
While the cake cools, prepare the mousse. In a small saucepan, heat 1/2 cup of heavy cream over medium heat until it begins to simmer.
Remove from heat and add the chopped chocolate and instant coffee granules (if using). Stir until the chocolate is completely melted and the mixture is smooth.
In a separate bowl, whip the remaining 1/2 cup of heavy cream with the powdered sugar until soft peaks form.
Gently fold the whipped cream into the chocolate mixture until fully combined.
Once the cake is completely cool, slice it in half horizontally to create two layers.
Spread half of the chocolate mousse over the bottom layer of the cake, then place the top layer back on.
Top the cake with the remaining mousse and smooth it out. Refrigerate for at least 2 hours to set before serving.
