Follow these steps carefully, particularly the folding technique, to ensure your mousse is light and airy. The process is straightforward but requires a gentle hand.

Step 1: Prepare the Raspberry Coulis

1. In a small saucepan, combine the raspberries (fresh or frozen), granulated sugar, and fresh lemon juice.
2. Place the saucepan over medium heat. As the raspberries heat up, they will begin to break down. Use the back of a spoon or a potato masher to help crush them and release their juices.
3. Bring the mixture to a gentle simmer and cook for 5-7 minutes, stirring occasionally, until the berries are completely broken down and the sauce has thickened slightly.
4. Remove the saucepan from the heat. For an ultra-smooth, seedless coulis (highly recommended for an elegant texture), place a fine-mesh sieve over a clean bowl and pour the raspberry mixture into it.
5. Use a spatula or the back of a spoon to press the mixture through the sieve, scraping the underside of the sieve to get all the smooth purée. Discard the seeds left behind.
6. Set the finished coulis aside to cool completely to room temperature. You can speed this up by placing it in the refrigerator.

Step 2: Prepare the Mousse Base

1. First, bloom the gelatin. Pour the 2 tablespoons of cold water into a small, microwave-safe bowl. Sprinkle the powdered gelatin evenly over the top and let it sit for 5-10 minutes without stirring. It will absorb the water and become a thick, jelly-like mass.
2. Chop the high-quality white chocolate into small, even pieces. This will help it melt quickly and evenly.
3. In a medium, heatproof bowl, combine the chopped white chocolate with ½ cup (120ml) of the heavy cream.
4. Create a double boiler by setting the bowl over a saucepan of simmering water (ensure the bottom of the bowl does not touch the water). Stir gently and constantly until the white chocolate is completely melted and the mixture is smooth. Alternatively, you can melt it in the microwave in 20-second intervals, stirring well between each, until smooth. Be very careful not to overheat the chocolate.
5. Once the chocolate is melted, remove the bowl from the heat.
6. Take your bowl of bloomed gelatin and microwave it for 5-10 seconds, just until it turns back into a liquid. It should not be boiling hot.
7. Immediately pour the liquefied gelatin into the warm white chocolate mixture and whisk vigorously until it is fully incorporated and the mixture is perfectly smooth. Set this mixture aside to cool for about 10-15 minutes, until it is at room temperature. It should not be warm to the touch, as hot chocolate will deflate the whipped cream.

Step 3: Whip the Cream and Assemble the Mousse

1. In a large, chilled bowl, pour the remaining 1 ½ cups (360ml) of cold heavy whipping cream and the vanilla extract.
2. Using an electric hand mixer or a stand mixer with the whisk attachment, beat the cream on medium-high speed.
3. Continue beating until the cream forms stiff peaks. This means when you lift the beaters out of the cream, the peak that forms holds its shape and doesn’t flop over. Be careful not to over-beat, or you will start making butter.
4. Gently pour the cooled white chocolate and gelatin mixture into the bowl with the whipped cream.
5. This is the most critical step: folding. Using a large rubber spatula, gently fold the white chocolate mixture into the whipped cream. Cut down through the middle of the bowl, scrape along the bottom, and bring the spatula up the side, turning the mixture over on itself. Rotate the bowl and repeat. Continue this gentle motion just until no white streaks remain. Do not stir or whisk, as this will deflate all the air you just whipped into the cream. The goal is to combine them while keeping the mixture as light and airy as possible.

Step 4: Layer and Chill

1. Prepare your serving glasses (martini glasses, small jars, or ramekins work well).
2. Begin layering. You can do this in any pattern you like. A classic method is to spoon a layer of the white chocolate mousse into the bottom of each glass.
3. Follow with a layer of the cooled raspberry coulis.
4. Add another layer of mousse, and finish with a final drizzle or dollop of coulis on top. You can also create beautiful swirls by dragging a skewer through the layers.
5. Cover the glasses with plastic wrap and transfer them to the refrigerator.
6. Chill for at least 6 hours, but preferably overnight. This time is crucial for the gelatin to set properly and for the flavors to meld together, resulting in the perfect mousse texture.