- With the rack in the middle position, preheat the oven to 350°F (180°C). Butter an 8-inch (20 cm) square baking pan and line the bottom with a sheet of parchment paper, letting the paper hang over two sides (see note).
- In a bowl, dissolve the espresso powder in the water. Let cool for 5 minutes. Add the yogurt and mix well.
- In another bowl, combine the flour, baking powder, baking soda and salt.
- In a third bowl, cream the butter and sugar with an electric mixer until light and fluffy. Whisk in the eggs, one at a time, until smooth. Add the dry ingredients alternating with the yogurt mixture. Pour into the prepared pan.
- Bake for 30 minutes or until a toothpick inserted in the centre of the cake comes out clean. Let cool completely on a wire rack, about 2 hours.
- In a bowl, whisk the cream with the sugar and vanilla seeds until firm peaks form.
- In another bowl, stir the chestnut cream with a spatula to loosen. Spread over the cake and top with the whipped cream. Lightly sprinkle with espresso powder, if desired. Slice into squares and serve.
The instant espresso powder can be replaced with 6 tbsp (90 ml) of brewed espresso. In this case, omit the espresso powder and hot water.