• Preheat the oven to 350 F (180 C) for conventional oven, 330 F (165 C) for convection oven
• Line one large baking pan or two baking pans if you don't have a large one, with parchment paper
• If you use two baking pan, position the rack at the upper 3rd and lower 3rd of the oven. If you bake one pan at a time, position the rack in the middle of the oven

Prepare the biscuit dough:

• Place the softened unsalted butter, salt, and powdered sugar in a mixing bowl. If you use a stand mixer, use a flat beater and beat until the sugar and butter are creamy. Then add the flour, custard powder, salt, and vanilla extract. Use the lowest speed to beat again until crumbly mixture forms. Then use your hands to gently push the crumbly mixture to form a soft non-sticky dough
• You can also do this manually by using a sturdy spatula to beat the butter, salt, and powdered sugar until creamy
  
• Then add custard powder, and vanilla extract and mix again to combine
  
• Add the flour and mix until crumbly mixture forms.
  
• Then use your hand to gently push the dough together. You should be able to form a soft dough. It shouldn't be sticky
  

Shape the dough:

• Pinch off about 15 grams of dough, about a tablespoon or so of the dough
  
• Roll them into balls
  
• Place them on a prepared baking pan, about 2 inches apart. You will get about 30 biscuits give and take
  
• Dip the tines of a fork in a flour and use while one hand is holding the cookie dough the other is holding a fork and gently press the tines of the fork on top of the cookie dough to create a pattern on top.
  
• Don't press it all the way down because it can cause the dough to crack on the edge. Just flatten a bit and enough to make the pattern deep enough with the fork
  

Baking:

• Place baking pan in the positioned racks. Bake for 10 minutes and then rotate the pan. If you bake two pans at the same time, rotate the pans from top to bottom, front to back. Continue baking for another 10-12 minutes. The biscuits won't take on much color at all. Do not overbake these biscuits

Cooling:

• Place the pan on a cooling rack. Let the biscuits cool on the pan for 10 minutes. They are very soft and fragile when they are just out of the oven. After 10 minutes, use an offset spatula and slide underneath the biscuit and transfer to a cooling rack to cool completely before sandwich them with buttercream

Prepare the buttercream:

• Put the softened butter, powdered sugar, pinch of salt, and lemon juice in a mixing bowl. Use a hand mixer or stand mixer to to whip the mixture until creamy and pale over medium-high speed
  
• You can add few drop of gel food coloring or flavor extract of your choice if you want. I divide the mixture into two. Add a drop or two of ube extract to one of them and leave the other in original flavor. This amount of buttercream is a satisfying amount for 15 biscuits.
  

Assembling:

• Transfer the buttercream to a piping bag fitted with 1-inch nozzle. You can use plain tip or starburst, etc.
• Place 15 biscuits, flat side facing up. Pipe the buttercream on the biscuits. It may seem like lots of buttercream, but trust me, you won't regret it. Place another biscuit on top to make a sandwich
  

Store:

• Place yo yo biscuits in an air-tight container for about 5 days. You want to keep them in a cool dry place. If it's really warm and humid where you are, keep them in the fridge for no more than one week. Keep in mind that the longer you keep them in the fridge, the drier the biscuits get
• For longer storage, I recommend freezing them. Simply put them in an air-tight freezer-friendly container and they can be kept for 2-3 weeks max. Simply thaw at room temperature before serving
• You can also store the unfilled biscuits separately in the freezer and keep the buttercream in the fridge for up to a week, or a month in the freezer. Just let the buttercream soften at room temperature before using and then assemble as sandwiches and serve on the same day if you want them "freshly" made