Preheat oven to 350°F. Line a 9x9-inch square pan with parchment paper leaving a 2-inch overhang at each side.
Place milk, almond extract and ⅓ cup of sugar in a medium saucepan over low heat for 3-4 minutes while stirring occasionally until the milk is warmed through and sugar is dissolved. Turn off the heat and let cool for 5-10 minutes.
Place flour, ground almonds, the remaining ¼ cup of sugar, baking powder, baking soda and salt in a large bowl and whisk to blend evenly. Add melted butter and eggs and begin whisking to break up the yolks. Gradually add the milk mixture to the flour mixture while whisking until it forms a smooth batter.
Pour the batter into the prepared pan and spread it out evenly. Bake for 20-25 minutes until the cake is golden and a skewer inserted into the center comes out clean. Transfer to a wire rack and let cool completely.
For the mousse, combine milk chocolate, water and espresso powder in a small saucepan over medium-low heat and stir until smooth. Remove from heat and place into a clean bowl. Stir in peanut butter and let cool to room temperature. Whip the cold cream until it forms soft peaks and fold it into the chocolate mixture. Spread it over cooled cake and chill for 1 hour. Dust with icing sugar and top with toasted sliced almonds before serving.