Preheat oven to 350 degrees F (175 degrees C). Line a rimmed baking sheet with parchment paper.
Combine eggs, sugar, and vanilla sugar in a bowl and beat with an electric blender until foamy. Add oil and orange soda and stir to combine.
Sift flour and baking powder together and slowly stir into batter until well combined. Spread batter on the prepared baking sheet.
Bake in the preheated oven until a toothpick inserted in the center of the cake comes out clean, about 30 minutes. Remove from oven and allow to cool completely, about 1 hour.
Sort chocolate-covered candy by the colors in your food color assortment, such as yellow, green, orange, blue, and red.
Cut cooled cake into evenly sized rectangles, about 1.5 x 3 inches in size, like oversized LEGO(R) pieces. Gauge the size in relationship to the chocolate-covered candy pieces; 8 pieces should fit onto each to resemble a large LEGO(R) piece.
Mix about 1/2 cup confectioners sugar with enough lemon juice in a bowl to make a thick icing. Color with a couple drops of food coloring so icing turns a deep color. Brush icing onto 2 or 3 cake pieces and arrange 8 chocolate-covered candy pieces with the matching color on top. Allow to set.
Continue with the next color and the next set of chocolate-covered candy pieces in the same color until all cake pieces are decorated. Allow to set.