Prepare the Coffee Base

1. Preheat the Oven: Start by preheating your oven to 350°F (175°C). Line an 8×8-inch square baking pan with parchment paper or lightly grease it to ensure easy removal later.
2. Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
3. Cream Butter and Sugar: In a separate large bowl, cream together the softened butter and granulated sugar until light and fluffy. This will take about 2-3 minutes using an electric mixer or by hand with a whisk.
4. Add Eggs and Flavorings: Beat in the eggs, one at a time, followed by the instant coffee granules, vanilla extract, and sour cream. Mix until everything is fully combined.
5. Combine with Dry Ingredients: Gradually add the dry ingredients to the wet ingredients, mixing just until incorporated. Be careful not to overmix, as this could make the bars dense.
6. Pour into Pan: Spread the batter evenly into the prepared baking pan, smoothing out the top with a spatula.

Prepare the Cappuccino Swirl

1. Make the Cappuccino Mixture: In a small saucepan, combine the heavy cream, instant coffee granules, and powdered sugar. Heat over low heat, stirring constantly until the coffee granules are dissolved and the mixture is smooth.
2. Add Cocoa and Vanilla: Stir in the cocoa powder and vanilla extract until everything is fully combined and the mixture is velvety smooth.
3. Swirl onto the Base: Drop spoonfuls of the cappuccino mixture onto the surface of the batter. Use a knife or skewer to gently swirl the coffee mixture into the batter, creating a marbled effect.

Bake the Bars

1. Bake: Place the pan in the preheated oven and bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs. The edges should be slightly golden brown, and the top should be firm to the touch.
2. Cool: Remove the pan from the oven and allow the bars to cool in the pan for about 10 minutes. Then, lift the bars out using the parchment paper (if you used it) and transfer them to a wire rack to cool completely.