Preheat oven to 350°F (175°C). In a stand mixer with a paddle attachment, cream butter and sugars on medium-high speed for 2 to 3 minutes until fluffy and pale yellow.

Scrape down the sides of the mixing bowl. On a lower speed, add the egg and mix until incorporated.

Increase speed to medium-high for 1 to 2 minutes until sugar granules fully dissolve and mixture is pale white.

On low speed, add flour, oats, baking powder, baking soda, and salt. Mix for 60-75 seconds until dough comes together. Dough will be a slightly fluffy, fatty mixture. Scrape down the sides of the bowl.

Grease and line a quarter sheet pan with parchment or a Silpat. Spread the oat cookie dough evenly, about 1/4 inch thick. Dough will not cover the entire pan.

Bake the oat cookie for 15 minutes. Cool completely before using.

In a stand mixer with a paddle attachment on low speed, mix the dry ingredients for the filling. Keep mixer on low throughout to avoid incorporating too much air.

Add melted butter and paddle until all dry ingredients are moist.

Add heavy cream and vanilla extract, mix until cream is fully incorporated. Scrape down the sides of the bowl.

Add egg yolks and paddle just to combine. Be careful not to aerate the mixture. Use immediately.

Preheat oven to 350°F (175°C). In a food processor, pulse the cooled oat cookie with brown sugar and salt until it resembles wet sand.

Transfer crumbs to a bowl. Knead in the melted butter until the mixture is moist enough to form a ball. Add an additional 1-1.5 tablespoons of butter if needed.

Divide the oat crust mixture evenly between two 10-inch pie tins.

Press the crust firmly into the bottom and up the sides of both pie tins. Use immediately or store wrapped in plastic at room temperature for up to 5 days or in the fridge for up to 2 weeks.

Place pie shells on a baking sheet. Divide the filling evenly between both crusts, filling about 3/4 full. Bake at 350°F (175°C) for 15 minutes, until golden brown and jiggly.

Open the oven door and reduce temperature to 325°F (160°C). Keep pies in oven while temperature adjusts. Once at 325°F, close door and bake for an additional 5 minutes.

Pies should still be jiggly in the center. If too jiggly, bake for another 5 minutes.

Remove from oven and cool on a wire rack. For a denser filling, chill in the fridge for at least 3 hours or overnight.

Just before serving, dust generously with confectioners' sugar.