Preheat the oven to 350°F (180°C). Line the bottom of a 9 by 13-inch (23 by 33 cm) baking pan with parchment paper and grease the sides of the pan with vegetable oil or baking spray.

In a large bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt. Add the water, plant milk, coconut oil or plant butter, lemon juice, and both extracts. Whisk just until incorporated. Transfer to the prepared pan and smooth the top. Bake until golden and springy when pressed lightly with a fingertip, 25 to 30 minutes. Let cool in the pan.

Use a long serrated knife to trim the sides of the cake so that they are even.

In a small saucepan, warm the apricot jam and water over medium-low heat until syrupy, about 2 minutes. Brush over the top of the cake.

Prepare the marzipan layer

Dust a clean work surface with powdered sugar and roll out the marzipan to an 8 by 12-inch (20 by 30 cm) rectangle. Lay the marzipan over the top of the cake. Use a rolling pin or your palms press down very gently, just enough to adhere the marzipan to the cake. Chill in the freezer for 10 minutes.

Gently melt the white chocolate in a double boiler. In a liquid measuring cup, stir together the hot water, agave syrup, and vanilla. When the chocolate has melted completely, alternate adding about one-quarter of the water mixture and one-quarter of the powdered sugar. Whisk continuously to incorporate. Remove from the heat.

Place a wire rack over a piece of parchment paper.

Cut the chilled cake into 18 (2 by 1½-inch / 5 by 4 cm) rectangles. Stick a fork in the bottom of one rectangle and then dip it into the icing. Turn it right-side up and place on the wire rack. Repeat. Chill for a few minutes in the freezer to allow the icing to set, then garnish with flowers or a few sliced almonds in the shape of a flower.