In a large bowl, use an electric mixer to beat butter and sugar until light and fluffy, scraping the sides of the bowl as needed.
Beat the egg lightly with a whisk. Add the egg, but reserve about a tablespoon of egg to brush over the top of the cake. Mix the remaining egg into the butter mixture.
Mix in the almond extract.
Add in the flour and baking powder. Mix just until combined. The dough will be stiff like sugar cookie dough.
Press evenly into the prepared pan.
Sprinkle with sliced almonds, then gently press them into the top of the cake with the palms of your hands.
Brush the tops with a thin layer of the reserved egg.
Bake in the preheated oven for about 27 minutes, or until the top is golden brown. Do not overbake!
Allow the cake to cool completely.
Although the cake is good warm from the oven, the flavors are best if you serve the cake the next day. Slice into wedges to serve.
Store leftovers in an airtight container.
This is a traditional Dutch Boterkoek.
It is great warm, but best if you let it sit for a day.
This cake freezes well for up to 2 months.
For the best texture, do not overbake.
You can double the recipe and bake in a 9x13 pan. It may take just a few minutes longer.
You can also bake the single recipe in an 8x8 pan for the same amount of time
The calories shown are based on the cake being cut into 8 pieces, with 1 serving being 1 slice of cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information.**