Preheat the oven to 350°F (180°C). Grease and line an 8-inch round cake pan with parchment paper.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.
In a separate bowl, combine the buttermilk, vegetable oil, egg, and vanilla extract. Pour the wet ingredients into the dry ingredients and mix until just combined.
In a small bowl, whisk the instant coffee powder and hot water until frothy and thick.
Pour the cake batter into the prepared pan. Drop spoonfuls of the whipped coffee mixture on top of the batter. Use a toothpick to swirl the coffee into the batter.
Bake the cake for 25-30 minutes or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
In a mixing bowl, whip the heavy cream and powdered sugar until stiff peaks form.
Spread the whipped cream over the cooled cake. Serve the Dalgona cake and enjoy!