For the custard:
Combine the zest, milk, and 1/4 cup sugar in a medium saucepan and heat over medium-low heat until just hot.
In a medium sized bowl, whisk together the egg yolks, the remaining 1/4 cup sugar, and the salt until well-combined, then whisk in the cornstarch and lemon juice.
Slowly whisk about 1/3 of the hot mixture into the yolk mixture. Pour the yolk mixture back into the saucepan of hot milk, whisking steadily, until the custard begins to thicken and bubble for one minute. Note: Helpful to have an instant thermometer. Egg-based puddings and custards can curdle if cooked beyond 185 degrees. We wanted the custard thick enough to hold a design on the cake so we shot for a smidge over 180 degrees. Once your custard gets to between 175-180, take it off the heat but do not strain immediately, give it a couple of minutes. The heat from the pot should push it to 180.
Strain the custard through a fine mesh into a clean bowl, and whisk in the butter until melted. Place a piece of plastic wrap directly on the surface of the custard and refrigerate for two hours.
For the cake:
Center an oven rack, and preheat the oven to 350 degrees. Grease a 9 inch round cake pan and line the bottom of the pan with parchment paper.
Sift together the flour, baking powder, and salt in a bowl, then whisk to combine well.
Using a stand mixer with a paddle attachment, combine the almond paste, butter, sugar, canola oil, and vanilla on low speed until blended. Increase the speed to high, and cream until very light and fluffy, about 5 to 7 minutes. Stop the mixer frequently to scrape the paddle and sides of the bowl.
Blend in the eggs one at a time, adding the next one as soon as the previous one has disappeared into the batter. With the mixer on low speed, add the flour in three parts, alternating with the buttermilk in two parts, begin and end with flour. After each addition, mix until just barely blended, and scrape the sides of the bowl.
Pour the batter into the prepared pan and spread it evenly. Rap the pan firmly on the counter to release any air bubbles. Place the pan in the center of the oven and bake until the cake is a deep golden color and a wooden pick comes out barely clean, about 42 to 45 minutes. The batter will fill the pan but shouldn't overflow while baking. BUT, just in case, you might want to put foil underneath the pan.
Cool the cake in pan on a wire rack for 30 minutes. After 30 minutes, flip the cake out onto the wire rack, remove the parchment paper, and flip the cake back to right side up. Cool to room temperature.
Place cake on a serving plate and frost the sides of the cake lightly with the lemon custard. Frost the top of the cake with remaining custard, then let the cake set in the refrigerator for about 30 minutes. Any leftover cake keeps in an airtight container in the refrigerator for up to three days.
